[PATCHSET v9 sched_ext/for-6.19] Add a deadline server for sched_ext tasks

[PATCHSET v9 sched_ext/for-6.19] Add a deadline server for sched_ext tasks

[Andrea Righi]

sched_ext tasks can be starved by long-running RT tasks, especially since
RT throttling was replaced by deadline servers to boost only SCHED_NORMAL
tasks.

Several users in the community have reported issues with RT stalling
sched_ext tasks. This is fairly common on distributions or environments
where applications like video compositors, audio services, etc. run as RT
tasks by default.

Example trace (showing a per-CPU kthread stalled due to the sway Wayland
compositor running as an RT task):

 runnable task stall (kworker/0:0[106377] failed to run for 5.043s)
 ...
 CPU 0   : nr_run=3 flags=0xd cpu_rel=0 ops_qseq=20646200 pnt_seq=45388738
           curr=sway[994] class=rt_sched_class
   R kworker/0:0[106377] -5043ms
       scx_state/flags=3/0x1 dsq_flags=0x0 ops_state/qseq=0/0
       sticky/holding_cpu=-1/-1 dsq_id=0x8000000000000002 dsq_vtime=0 slice=20000000
       cpus=01

This is often perceived as a bug in the BPF schedulers, but in reality they
can't do much: RT tasks run outside their control and can potentially
consume 100% of the CPU bandwidth.

Fix this by adding a sched_ext deadline server as well so that sched_ext
tasks are also boosted and do not suffer starvation.

Two kselftests are also provided to verify the starvation fixes and
bandwidth allocation is correct.

This patchset is also available in the following git branch:

 git://git.kernel.org/pub/scm/linux/kernel/git/arighi/linux.git scx-dl-server

Changes in v9:
 - Drop the ->balance() logic as its functionality is now integrated into
   ->pick_task(), allowing dl_server to call pick_task_scx() directly
 - Link to v8:
   https://lore.kernel.org/all/20250903095008.162049-1-arighi@nvidia.com/

Changes in v8:
 - Add tj's patch to de-couple balance and pick_task and avoid changing
   sched/core callbacks to propagate @rf
 - Simplify dl_se->dl_server check (suggested by PeterZ)
 - Small coding style fixes in the kselftests
 - Link to v7: https://lore.kernel.org/all/20250809184800.129831-1-joelagnelf@nvidia.com/

Changes in v7:
 - Rebased to Linus master
 - Link to v6: https://lore.kernel.org/all/20250702232944.3221001-1-joelagnelf@nvidia.com/

Changes in v6:
 - Added Acks to few patches
 - Fixes to few nits suggested by Tejun
 - Link to v5: https://lore.kernel.org/all/20250620203234.3349930-1-joelagnelf@nvidia.com/

Changes in v5:
 - Added a kselftest (total_bw) to sched_ext to verify bandwidth values
   from debugfs
 - Address comment from Andrea about redundant rq clock invalidation
 - Link to v4: https://lore.kernel.org/all/20250617200523.1261231-1-joelagnelf@nvidia.com/

Changes in v4:
 - Fixed issues with hotplugged CPUs having their DL server bandwidth
   altered due to loading SCX
 - Fixed other issues
 - Rebased on Linus master
 - All sched_ext kselftests reliably pass now, also verified that the
   total_bw in debugfs (CONFIG_SCHED_DEBUG) is conserved with these patches
 - Link to v3: https://lore.kernel.org/all/20250613051734.4023260-1-joelagnelf@nvidia.com/

Changes in v3:
 - Removed code duplication in debugfs. Made ext interface separate
 - Fixed issue where rq_lock_irqsave was not used in the relinquish patch
 - Fixed running bw accounting issue in dl_server_remove_params
 - Link to v2: https://lore.kernel.org/all/20250602180110.816225-1-joelagnelf@nvidia.com/

Changes in v2:
 - Fixed a hang related to using rq_lock instead of rq_lock_irqsave
 - Added support to remove BW of DL servers when they are switched to/from EXT
 - Link to v1: https://lore.kernel.org/all/20250315022158.2354454-1-joelagnelf@nvidia.com/

Andrea Righi (5):
      sched/deadline: Add support to remove DL server's bandwidth contribution
      sched/deadline: Account ext server bandwidth
      sched/deadline: Allow to initialize DL server when needed
      sched_ext: Selectively enable ext and fair DL servers
      selftests/sched_ext: Add test for sched_ext dl_server

Joel Fernandes (9):
      sched/debug: Fix updating of ppos on server write ops
      sched/debug: Stop and start server based on if it was active
      sched/deadline: Clear the defer params
      sched/deadline: Return EBUSY if dl_bw_cpus is zero
      sched: Add a server arg to dl_server_update_idle_time()
      sched_ext: Add a DL server for sched_ext tasks
      sched/debug: Add support to change sched_ext server params
      sched/deadline: Fix DL server crash in inactive_timer callback
      selftests/sched_ext: Add test for DL server total_bw consistency

 kernel/sched/core.c                              |   3 +
 kernel/sched/deadline.c                          | 138 ++++++++---
 kernel/sched/debug.c                             | 163 ++++++++++---
 kernel/sched/ext.c                               | 148 +++++++++++-
 kernel/sched/fair.c                              |   2 +-
 kernel/sched/idle.c                              |   2 +-
 kernel/sched/sched.h                             |   7 +-
 kernel/sched/topology.c                          |   5 +
 tools/testing/selftests/sched_ext/Makefile       |   2 +
 tools/testing/selftests/sched_ext/rt_stall.bpf.c |  23 ++
 tools/testing/selftests/sched_ext/rt_stall.c     | 214 +++++++++++++++++
 tools/testing/selftests/sched_ext/total_bw.c     | 281 +++++++++++++++++++++++
 12 files changed, 918 insertions(+), 70 deletions(-)
 create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c
 create mode 100644 tools/testing/selftests/sched_ext/total_bw.c

[PATCH 01/14] sched/debug: Fix updating of ppos on server write ops

[Andrea Righi]

From: Joel Fernandes <joelagnelf@nvidia.com>

Updating "ppos" on error conditions does not make much sense. The pattern
is to return the error code directly without modifying the position, or
modify the position on success and return the number of bytes written.

Since on success, the return value of apply is 0, there is no point in
modifying ppos either. Fix it by removing all this and just returning
error code or number of bytes written on success.

Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/sched/debug.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 02e16b70a7901..6cf9be6eea49a 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -345,8 +345,8 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
 	long cpu = (long) ((struct seq_file *) filp->private_data)->private;
 	struct rq *rq = cpu_rq(cpu);
 	u64 runtime, period;
+	int retval = 0;
 	size_t err;
-	int retval;
 	u64 value;
 
 	err = kstrtoull_from_user(ubuf, cnt, 10, &value);
@@ -380,8 +380,6 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
 		dl_server_stop(&rq->fair_server);
 
 		retval = dl_server_apply_params(&rq->fair_server, runtime, period, 0);
-		if (retval)
-			cnt = retval;
 
 		if (!runtime)
 			printk_deferred("Fair server disabled in CPU %d, system may crash due to starvation.\n",
@@ -389,6 +387,9 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
 
 		if (rq->cfs.h_nr_queued)
 			dl_server_start(&rq->fair_server);
+
+		if (retval < 0)
+			return retval;
 	}
 
 	*ppos += cnt;
-- 
2.51.0

Re: [PATCH 01/14] sched/debug: Fix updating of ppos on server write ops

[Juri Lelli]

Hi!

On 17/10/25 11:25, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@nvidia.com>
> 
> Updating "ppos" on error conditions does not make much sense. The pattern
> is to return the error code directly without modifying the position, or
> modify the position on success and return the number of bytes written.
> 
> Since on success, the return value of apply is 0, there is no point in
> modifying ppos either. Fix it by removing all this and just returning
> error code or number of bytes written on success.
> 
> Acked-by: Tejun Heo <tj@kernel.org>
> Reviewed-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

Reviewed-by: Juri Lelli <juri.lelli@redhat.com>

Thanks,
Juri

[PATCH 02/14] sched/debug: Stop and start server based on if it was active

[Andrea Righi]

From: Joel Fernandes <joelagnelf@nvidia.com>

Currently the DL server interface for applying parameters checks
CFS-internals to identify if the server is active. This is error-prone
and makes it difficult when adding new servers in the future.

Fix it, by using dl_server_active() which is also used by the DL server
code to determine if the DL server was started.

Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/sched/debug.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 6cf9be6eea49a..e71f6618c1a6a 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -354,6 +354,8 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
 		return err;
 
 	scoped_guard (rq_lock_irqsave, rq) {
+		bool is_active;
+
 		runtime  = rq->fair_server.dl_runtime;
 		period = rq->fair_server.dl_period;
 
@@ -376,8 +378,11 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
 			return  -EINVAL;
 		}
 
-		update_rq_clock(rq);
-		dl_server_stop(&rq->fair_server);
+		is_active = dl_server_active(&rq->fair_server);
+		if (is_active) {
+			update_rq_clock(rq);
+			dl_server_stop(&rq->fair_server);
+		}
 
 		retval = dl_server_apply_params(&rq->fair_server, runtime, period, 0);
 
@@ -385,7 +390,7 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
 			printk_deferred("Fair server disabled in CPU %d, system may crash due to starvation.\n",
 					cpu_of(rq));
 
-		if (rq->cfs.h_nr_queued)
+		if (is_active)
 			dl_server_start(&rq->fair_server);
 
 		if (retval < 0)
-- 
2.51.0

Re: [PATCH 02/14] sched/debug: Stop and start server based on if it was active

[Juri Lelli]

Hi!

On 17/10/25 11:25, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@nvidia.com>
> 
> Currently the DL server interface for applying parameters checks
> CFS-internals to identify if the server is active. This is error-prone
> and makes it difficult when adding new servers in the future.
> 
> Fix it, by using dl_server_active() which is also used by the DL server
> code to determine if the DL server was started.
> 
> Acked-by: Tejun Heo <tj@kernel.org>
> Reviewed-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> ---
>  kernel/sched/debug.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 6cf9be6eea49a..e71f6618c1a6a 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -354,6 +354,8 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
>  		return err;
>  
>  	scoped_guard (rq_lock_irqsave, rq) {
> +		bool is_active;
> +
>  		runtime  = rq->fair_server.dl_runtime;
>  		period = rq->fair_server.dl_period;
>  
> @@ -376,8 +378,11 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
>  			return  -EINVAL;
>  		}
>  
> -		update_rq_clock(rq);
> -		dl_server_stop(&rq->fair_server);
> +		is_active = dl_server_active(&rq->fair_server);
> +		if (is_active) {
> +			update_rq_clock(rq);
> +			dl_server_stop(&rq->fair_server);
> +		}

Won't this reintroduce what bb4700adc3abe ("sched/deadline: Always stop
dl-server before changing parameters") fixed?

Thanks,
Juri

Re: [PATCH 02/14] sched/debug: Stop and start server based on if it was active

[Juri Lelli]

On 20/10/25 11:12, Juri Lelli wrote:
> Hi!
> 
> On 17/10/25 11:25, Andrea Righi wrote:
> > From: Joel Fernandes <joelagnelf@nvidia.com>
> > 
> > Currently the DL server interface for applying parameters checks
> > CFS-internals to identify if the server is active. This is error-prone
> > and makes it difficult when adding new servers in the future.
> > 
> > Fix it, by using dl_server_active() which is also used by the DL server
> > code to determine if the DL server was started.
> > 
> > Acked-by: Tejun Heo <tj@kernel.org>
> > Reviewed-by: Andrea Righi <arighi@nvidia.com>
> > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > ---
> >  kernel/sched/debug.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> > index 6cf9be6eea49a..e71f6618c1a6a 100644
> > --- a/kernel/sched/debug.c
> > +++ b/kernel/sched/debug.c
> > @@ -354,6 +354,8 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
> >  		return err;
> >  
> >  	scoped_guard (rq_lock_irqsave, rq) {
> > +		bool is_active;
> > +
> >  		runtime  = rq->fair_server.dl_runtime;
> >  		period = rq->fair_server.dl_period;
> >  
> > @@ -376,8 +378,11 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
> >  			return  -EINVAL;
> >  		}
> >  
> > -		update_rq_clock(rq);
> > -		dl_server_stop(&rq->fair_server);
> > +		is_active = dl_server_active(&rq->fair_server);
> > +		if (is_active) {
> > +			update_rq_clock(rq);
> > +			dl_server_stop(&rq->fair_server);
> > +		}
> 
> Won't this reintroduce what bb4700adc3abe ("sched/deadline: Always stop
> dl-server before changing parameters") fixed?

Ah, OK. It looks like it doesn't, as dl_server_active() is the correct
thing to use/check. Also in case the server was not active is should be
enqueued in defer mode, so no need to have an updated clock just yet.

Reviewed-by: Juri Lelli <juri.lelli@redhat.com>

Thanks,
Juri

[PATCH 03/14] sched/deadline: Clear the defer params

[Andrea Righi]

From: Joel Fernandes <joelagnelf@nvidia.com>

The defer params were not cleared in __dl_clear_params. Clear them.

Without this is some of my test cases are flaking and the DL timer is
not starting correctly AFAICS.

Fixes: a110a81c52a9 ("sched/deadline: Deferrable dl server")
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/sched/deadline.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 48357d4609bf9..4aefb34a1d38b 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -3387,6 +3387,9 @@ static void __dl_clear_params(struct sched_dl_entity *dl_se)
 	dl_se->dl_non_contending	= 0;
 	dl_se->dl_overrun		= 0;
 	dl_se->dl_server		= 0;
+	dl_se->dl_defer			= 0;
+	dl_se->dl_defer_running		= 0;
+	dl_se->dl_defer_armed		= 0;
 
 #ifdef CONFIG_RT_MUTEXES
 	dl_se->pi_se			= dl_se;
-- 
2.51.0

[PATCH 04/14] sched/deadline: Return EBUSY if dl_bw_cpus is zero

[Andrea Righi]

From: Joel Fernandes <joelagnelf@nvidia.com>

Hotplugged CPUs coming online do an enqueue but are not a part of any
root domain containing cpu_active() CPUs. So in this case, don't mess
with accounting and we can retry later. Without this patch, we see
crashes with sched_ext selftest's hotplug test due to divide by zero.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/sched/deadline.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 4aefb34a1d38b..f2f5b1aea8e2b 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1665,7 +1665,12 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
 	cpus = dl_bw_cpus(cpu);
 	cap = dl_bw_capacity(cpu);
 
-	if (__dl_overflow(dl_b, cap, old_bw, new_bw))
+	/*
+	 * Hotplugged CPUs coming online do an enqueue but are not a part of any
+	 * root domain containing cpu_active() CPUs. So in this case, don't mess
+	 * with accounting and we can retry later.
+	 */
+	if (!cpus || __dl_overflow(dl_b, cap, old_bw, new_bw))
 		return -EBUSY;
 
 	if (init) {
-- 
2.51.0

Re: [PATCH 04/14] sched/deadline: Return EBUSY if dl_bw_cpus is zero

[Juri Lelli]

Hi!

On 17/10/25 11:25, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@nvidia.com>
> 
> Hotplugged CPUs coming online do an enqueue but are not a part of any
> root domain containing cpu_active() CPUs. So in this case, don't mess
> with accounting and we can retry later. Without this patch, we see
> crashes with sched_ext selftest's hotplug test due to divide by zero.
> 
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> ---
>  kernel/sched/deadline.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 4aefb34a1d38b..f2f5b1aea8e2b 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1665,7 +1665,12 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
>  	cpus = dl_bw_cpus(cpu);
>  	cap = dl_bw_capacity(cpu);
>  
> -	if (__dl_overflow(dl_b, cap, old_bw, new_bw))
> +	/*
> +	 * Hotplugged CPUs coming online do an enqueue but are not a part of any
> +	 * root domain containing cpu_active() CPUs. So in this case, don't mess
> +	 * with accounting and we can retry later.

Later when? It seems a little vague. :)

Thanks,
Juri

Re: [PATCH 04/14] sched/deadline: Return EBUSY if dl_bw_cpus is zero

[Andrea Righi]

On Mon, Oct 20, 2025 at 11:49:51AM +0200, Juri Lelli wrote:
> Hi!
> 
> On 17/10/25 11:25, Andrea Righi wrote:
> > From: Joel Fernandes <joelagnelf@nvidia.com>
> > 
> > Hotplugged CPUs coming online do an enqueue but are not a part of any
> > root domain containing cpu_active() CPUs. So in this case, don't mess
> > with accounting and we can retry later. Without this patch, we see
> > crashes with sched_ext selftest's hotplug test due to divide by zero.
> > 
> > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > ---
> >  kernel/sched/deadline.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 4aefb34a1d38b..f2f5b1aea8e2b 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1665,7 +1665,12 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
> >  	cpus = dl_bw_cpus(cpu);
> >  	cap = dl_bw_capacity(cpu);
> >  
> > -	if (__dl_overflow(dl_b, cap, old_bw, new_bw))
> > +	/*
> > +	 * Hotplugged CPUs coming online do an enqueue but are not a part of any
> > +	 * root domain containing cpu_active() CPUs. So in this case, don't mess
> > +	 * with accounting and we can retry later.
> 
> Later when? It seems a little vague. :)

Yeah, this comment is actually incorrect, we're not "retrying later"
anymore (we used to do that in a previous version), now the params are
applied via:

  ext.c:handle_hotplug() -> dl_server_on() -> dl_server_apply_params()

Or via scx_enable() when an scx scheduler is loaded. So, I'm wondering if
this condition is still needed. Will do some tests.

Thanks!
-Andrea

Re: [PATCH 04/14] sched/deadline: Return EBUSY if dl_bw_cpus is zero

[Andrea Righi]

On Mon, Oct 20, 2025 at 03:38:12PM +0200, Andrea Righi wrote:
> On Mon, Oct 20, 2025 at 11:49:51AM +0200, Juri Lelli wrote:
> > Hi!
> > 
> > On 17/10/25 11:25, Andrea Righi wrote:
> > > From: Joel Fernandes <joelagnelf@nvidia.com>
> > > 
> > > Hotplugged CPUs coming online do an enqueue but are not a part of any
> > > root domain containing cpu_active() CPUs. So in this case, don't mess
> > > with accounting and we can retry later. Without this patch, we see
> > > crashes with sched_ext selftest's hotplug test due to divide by zero.
> > > 
> > > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > > ---
> > >  kernel/sched/deadline.c | 7 ++++++-
> > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > index 4aefb34a1d38b..f2f5b1aea8e2b 100644
> > > --- a/kernel/sched/deadline.c
> > > +++ b/kernel/sched/deadline.c
> > > @@ -1665,7 +1665,12 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
> > >  	cpus = dl_bw_cpus(cpu);
> > >  	cap = dl_bw_capacity(cpu);
> > >  
> > > -	if (__dl_overflow(dl_b, cap, old_bw, new_bw))
> > > +	/*
> > > +	 * Hotplugged CPUs coming online do an enqueue but are not a part of any
> > > +	 * root domain containing cpu_active() CPUs. So in this case, don't mess
> > > +	 * with accounting and we can retry later.
> > 
> > Later when? It seems a little vague. :)
> 
> Yeah, this comment is actually incorrect, we're not "retrying later"
> anymore (we used to do that in a previous version), now the params are
> applied via:
> 
>   ext.c:handle_hotplug() -> dl_server_on() -> dl_server_apply_params()
> 
> Or via scx_enable() when an scx scheduler is loaded. So, I'm wondering if
> this condition is still needed. Will do some tests.

Looks like I can't reproduce the error with the hotplug kselftest anymore
(and it was happening pretty quickly).

Then I guess we can drop this patch or maybe add a WARN_ON_ONCE(!cpus) just
to safe?

Thanks,
-Andrea

Re: [PATCH 04/14] sched/deadline: Return EBUSY if dl_bw_cpus is zero

[Juri Lelli]

On 20/10/25 16:03, Andrea Righi wrote:
> On Mon, Oct 20, 2025 at 03:38:12PM +0200, Andrea Righi wrote:
> > On Mon, Oct 20, 2025 at 11:49:51AM +0200, Juri Lelli wrote:
> > > Hi!
> > > 
> > > On 17/10/25 11:25, Andrea Righi wrote:
> > > > From: Joel Fernandes <joelagnelf@nvidia.com>
> > > > 
> > > > Hotplugged CPUs coming online do an enqueue but are not a part of any
> > > > root domain containing cpu_active() CPUs. So in this case, don't mess
> > > > with accounting and we can retry later. Without this patch, we see
> > > > crashes with sched_ext selftest's hotplug test due to divide by zero.
> > > > 
> > > > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > > > ---
> > > >  kernel/sched/deadline.c | 7 ++++++-
> > > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > > index 4aefb34a1d38b..f2f5b1aea8e2b 100644
> > > > --- a/kernel/sched/deadline.c
> > > > +++ b/kernel/sched/deadline.c
> > > > @@ -1665,7 +1665,12 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
> > > >  	cpus = dl_bw_cpus(cpu);
> > > >  	cap = dl_bw_capacity(cpu);
> > > >  
> > > > -	if (__dl_overflow(dl_b, cap, old_bw, new_bw))
> > > > +	/*
> > > > +	 * Hotplugged CPUs coming online do an enqueue but are not a part of any
> > > > +	 * root domain containing cpu_active() CPUs. So in this case, don't mess
> > > > +	 * with accounting and we can retry later.
> > > 
> > > Later when? It seems a little vague. :)
> > 
> > Yeah, this comment is actually incorrect, we're not "retrying later"
> > anymore (we used to do that in a previous version), now the params are
> > applied via:
> > 
> >   ext.c:handle_hotplug() -> dl_server_on() -> dl_server_apply_params()
> > 
> > Or via scx_enable() when an scx scheduler is loaded. So, I'm wondering if
> > this condition is still needed. Will do some tests.
> 
> Looks like I can't reproduce the error with the hotplug kselftest anymore
> (and it was happening pretty quickly).
> 
> Then I guess we can drop this patch or maybe add a WARN_ON_ONCE(!cpus) just
> to safe?

WARN_ON_ONCE() works for me.

Thanks!

[PATCH 05/14] sched: Add a server arg to dl_server_update_idle_time()

[Andrea Righi]

From: Joel Fernandes <joelagnelf@nvidia.com>

Since we are adding more servers, make dl_server_update_idle_time()
accept a server argument than a specific server.

Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/sched/deadline.c | 15 ++++++++-------
 kernel/sched/fair.c     |  2 +-
 kernel/sched/idle.c     |  2 +-
 kernel/sched/sched.h    |  3 ++-
 4 files changed, 12 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index f2f5b1aea8e2b..0680e0186577a 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1543,26 +1543,27 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
  * as time available for the fair server, avoiding a penalty for the
  * rt scheduler that did not consumed that time.
  */
-void dl_server_update_idle_time(struct rq *rq, struct task_struct *p)
+void dl_server_update_idle_time(struct rq *rq, struct task_struct *p,
+			       struct sched_dl_entity *rq_dl_server)
 {
 	s64 delta_exec;
 
-	if (!rq->fair_server.dl_defer)
+	if (!rq_dl_server->dl_defer)
 		return;
 
 	/* no need to discount more */
-	if (rq->fair_server.runtime < 0)
+	if (rq_dl_server->runtime < 0)
 		return;
 
 	delta_exec = rq_clock_task(rq) - p->se.exec_start;
 	if (delta_exec < 0)
 		return;
 
-	rq->fair_server.runtime -= delta_exec;
+	rq_dl_server->runtime -= delta_exec;
 
-	if (rq->fair_server.runtime < 0) {
-		rq->fair_server.dl_defer_running = 0;
-		rq->fair_server.runtime = 0;
+	if (rq_dl_server->runtime < 0) {
+		rq_dl_server->dl_defer_running = 0;
+		rq_dl_server->runtime = 0;
 	}
 
 	p->se.exec_start = rq_clock_task(rq);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2554055c1ba13..562cdd253678a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6999,7 +6999,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	if (!rq_h_nr_queued && rq->cfs.h_nr_queued) {
 		/* Account for idle runtime */
 		if (!rq->nr_running)
-			dl_server_update_idle_time(rq, rq->curr);
+			dl_server_update_idle_time(rq, rq->curr, &rq->fair_server);
 		dl_server_start(&rq->fair_server);
 	}
 
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 7fa0b593bcff7..60a19ea9bdbb7 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -454,7 +454,7 @@ static void wakeup_preempt_idle(struct rq *rq, struct task_struct *p, int flags)
 
 static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct task_struct *next)
 {
-	dl_server_update_idle_time(rq, prev);
+	dl_server_update_idle_time(rq, prev, &rq->fair_server);
 	scx_update_idle(rq, false, true);
 }
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 63ffb3eafd05d..fa2fb64c1f3bf 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -412,7 +412,8 @@ extern void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
 extern void sched_init_dl_servers(void);
 
 extern void dl_server_update_idle_time(struct rq *rq,
-		    struct task_struct *p);
+		    struct task_struct *p,
+		    struct sched_dl_entity *rq_dl_server);
 extern void fair_server_init(struct rq *rq);
 extern void __dl_server_attach_root(struct sched_dl_entity *dl_se, struct rq *rq);
 extern int dl_server_apply_params(struct sched_dl_entity *dl_se,
-- 
2.51.0

Re: [PATCH 05/14] sched: Add a server arg to dl_server_update_idle_time()

[Juri Lelli]

Hi!

On 17/10/25 11:25, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@nvidia.com>
> 
> Since we are adding more servers, make dl_server_update_idle_time()
> accept a server argument than a specific server.

Nit,                      ^ rather?

> Reviewed-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

Acked-by: Juri Lelli <juri.lelli@redhat.com>

Thanks,
Juri

Re: [PATCH 05/14] sched: Add a server arg to dl_server_update_idle_time()

[Peter Zijlstra]

On Fri, Oct 17, 2025 at 11:25:52AM +0200, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@nvidia.com>
> 
> Since we are adding more servers, make dl_server_update_idle_time()
> accept a server argument than a specific server.
> 
> Reviewed-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

Can you run s/rq_dl_server/dl_server/g on the thing please?

[PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

[Andrea Righi]

From: Joel Fernandes <joelagnelf@nvidia.com>

sched_ext currently suffers starvation due to RT. The same workload when
converted to EXT can get zero runtime if RT is 100% running, causing EXT
processes to stall. Fix it by adding a DL server for EXT.

A kselftest is also provided later to verify:

./runner -t rt_stall
===== START =====
TEST: rt_stall
DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks
OUTPUT:
TAP version 13
1..1
ok 1 PASS: CFS task got more than 4.00% of runtime

[ arighi: drop ->balance() now that pick_task() has an rf argument ]

Cc: Luigi De Matteis <ldematteis123@gmail.com>
Co-developed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/sched/core.c     |  3 +++
 kernel/sched/deadline.c |  2 +-
 kernel/sched/ext.c      | 51 +++++++++++++++++++++++++++++++++++++++--
 kernel/sched/sched.h    |  2 ++
 4 files changed, 55 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 096e8d03d85e7..31a9c9381c63f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8679,6 +8679,9 @@ void __init sched_init(void)
 		hrtick_rq_init(rq);
 		atomic_set(&rq->nr_iowait, 0);
 		fair_server_init(rq);
+#ifdef CONFIG_SCHED_CLASS_EXT
+		ext_server_init(rq);
+#endif
 
 #ifdef CONFIG_SCHED_CORE
 		rq->core = rq;
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0680e0186577a..3c1fd2190949e 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1504,7 +1504,7 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
 	 * The fair server (sole dl_server) does not account for real-time
 	 * workload because it is running fair work.
 	 */
-	if (dl_se == &rq->fair_server)
+	if (dl_se->dl_server)
 		return;
 
 #ifdef CONFIG_RT_GROUP_SCHED
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index adff739b396ce..bc2aaa3236fd4 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -881,6 +881,9 @@ static void update_curr_scx(struct rq *rq)
 		if (!curr->scx.slice)
 			touch_core_sched(rq, curr);
 	}
+
+	if (dl_server_active(&rq->ext_server))
+		dl_server_update(&rq->ext_server, delta_exec);
 }
 
 static bool scx_dsq_priq_less(struct rb_node *node_a,
@@ -1388,6 +1391,15 @@ static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int enq_flags
 	if (enq_flags & SCX_ENQ_WAKEUP)
 		touch_core_sched(rq, p);
 
+	if (rq->scx.nr_running == 1) {
+		/* Account for idle runtime */
+		if (!rq->nr_running)
+			dl_server_update_idle_time(rq, rq->curr, &rq->ext_server);
+
+		/* Start dl_server if this is the first task being enqueued */
+		dl_server_start(&rq->ext_server);
+	}
+
 	do_enqueue_task(rq, p, enq_flags, sticky_cpu);
 out:
 	rq->scx.flags &= ~SCX_RQ_IN_WAKEUP;
@@ -1487,6 +1499,11 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int deq_flags
 	sub_nr_running(rq, 1);
 
 	dispatch_dequeue(rq, p);
+
+	/* Stop the server if this was the last task */
+	if (rq->scx.nr_running == 0)
+		dl_server_stop(&rq->ext_server);
+
 	return true;
 }
 
@@ -2987,6 +3004,15 @@ static void switching_to_scx(struct rq *rq, struct task_struct *p)
 static void switched_from_scx(struct rq *rq, struct task_struct *p)
 {
 	scx_disable_task(p);
+
+	/*
+	 * After class switch, if the DL server is still active, restart it so
+	 * that DL timers will be queued, in case SCX switched to higher class.
+	 */
+	if (dl_server_active(&rq->ext_server)) {
+		dl_server_stop(&rq->ext_server);
+		dl_server_start(&rq->ext_server);
+	}
 }
 
 static void wakeup_preempt_scx(struct rq *rq, struct task_struct *p,int wake_flags) {}
@@ -6498,8 +6524,8 @@ __bpf_kfunc u32 scx_bpf_cpuperf_cur(s32 cpu)
  * relative scale between 0 and %SCX_CPUPERF_ONE. This determines how the
  * schedutil cpufreq governor chooses the target frequency.
  *
- * The actual performance level chosen, CPU grouping, and the overhead and
- * latency of the operations are dependent on the hardware and cpufreq driver in
+ * The actual performance level chosen, CPU grouping, and the overhead and latency
+ * of the operations are dependent on the hardware and cpufreq driver in
  * use. Consult hardware and cpufreq documentation for more information. The
  * current performance level can be monitored using scx_bpf_cpuperf_cur().
  */
@@ -6874,6 +6900,27 @@ BTF_ID_FLAGS(func, scx_bpf_now)
 BTF_ID_FLAGS(func, scx_bpf_events, KF_TRUSTED_ARGS)
 BTF_KFUNCS_END(scx_kfunc_ids_any)
 
+/*
+ * Select the next task to run from the ext scheduling class.
+ */
+static struct task_struct *
+ext_server_pick_task(struct sched_dl_entity *dl_se, struct rq_flags *rf)
+{
+	return pick_task_scx(dl_se->rq, rf);
+}
+
+/*
+ * Initialize the ext server deadline entity.
+ */
+void ext_server_init(struct rq *rq)
+{
+	struct sched_dl_entity *dl_se = &rq->ext_server;
+
+	init_dl_entity(dl_se);
+
+	dl_server_init(dl_se, rq, ext_server_pick_task);
+}
+
 static const struct btf_kfunc_id_set scx_kfunc_set_any = {
 	.owner			= THIS_MODULE,
 	.set			= &scx_kfunc_ids_any,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index fa2fb64c1f3bf..55f8fbb306517 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -415,6 +415,7 @@ extern void dl_server_update_idle_time(struct rq *rq,
 		    struct task_struct *p,
 		    struct sched_dl_entity *rq_dl_server);
 extern void fair_server_init(struct rq *rq);
+extern void ext_server_init(struct rq *rq);
 extern void __dl_server_attach_root(struct sched_dl_entity *dl_se, struct rq *rq);
 extern int dl_server_apply_params(struct sched_dl_entity *dl_se,
 		    u64 runtime, u64 period, bool init);
@@ -1153,6 +1154,7 @@ struct rq {
 #endif
 
 	struct sched_dl_entity	fair_server;
+	struct sched_dl_entity	ext_server;
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
 	/* list of leaf cfs_rq on this CPU: */
-- 
2.51.0

Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

[Tejun Heo]

On Fri, Oct 17, 2025 at 11:25:53AM +0200, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@nvidia.com>
> 
> sched_ext currently suffers starvation due to RT. The same workload when
> converted to EXT can get zero runtime if RT is 100% running, causing EXT
> processes to stall. Fix it by adding a DL server for EXT.
> 
> A kselftest is also provided later to verify:
> 
> ./runner -t rt_stall
> ===== START =====
> TEST: rt_stall
> DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks
> OUTPUT:
> TAP version 13
> 1..1
> ok 1 PASS: CFS task got more than 4.00% of runtime
> 
> [ arighi: drop ->balance() now that pick_task() has an rf argument ]
> 
> Cc: Luigi De Matteis <ldematteis123@gmail.com>
> Co-developed-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>

I don't see anything preventing this to come after patch 11 so that all
sched_ext changes are at the end. Am I correct? That'd make applying the
patches easier. All the debug and deadline changes can be applied to
sched/core and I can pull that and apply sched_ext changes on top.

Thanks.

-- 
tejun

Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

[Andrea Righi]

On Fri, Oct 17, 2025 at 05:40:12AM -1000, Tejun Heo wrote:
> On Fri, Oct 17, 2025 at 11:25:53AM +0200, Andrea Righi wrote:
> > From: Joel Fernandes <joelagnelf@nvidia.com>
> > 
> > sched_ext currently suffers starvation due to RT. The same workload when
> > converted to EXT can get zero runtime if RT is 100% running, causing EXT
> > processes to stall. Fix it by adding a DL server for EXT.
> > 
> > A kselftest is also provided later to verify:
> > 
> > ./runner -t rt_stall
> > ===== START =====
> > TEST: rt_stall
> > DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks
> > OUTPUT:
> > TAP version 13
> > 1..1
> > ok 1 PASS: CFS task got more than 4.00% of runtime
> > 
> > [ arighi: drop ->balance() now that pick_task() has an rf argument ]
> > 
> > Cc: Luigi De Matteis <ldematteis123@gmail.com>
> > Co-developed-by: Andrea Righi <arighi@nvidia.com>
> > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> 
> I don't see anything preventing this to come after patch 11 so that all
> sched_ext changes are at the end. Am I correct? That'd make applying the
> patches easier. All the debug and deadline changes can be applied to
> sched/core and I can pull that and apply sched_ext changes on top.

Oh yes, we can definitely move this one. I'll reorder the patches.

Thanks,
-Andrea

Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

[Tejun Heo]

On Fri, Oct 17, 2025 at 11:25:53AM +0200, Andrea Righi wrote:
> +static struct task_struct *
> +ext_server_pick_task(struct sched_dl_entity *dl_se, struct rq_flags *rf)
> +{
> +	return pick_task_scx(dl_se->rq, rf);
> +}

I wonder whether we should tell pick_task_scx() to suppress the
rq_modified_above() test in this case as a fair or RT task being enqueued
has no reason to restart the picking process. While it will behave fine on
retry, it's probably useful to be explicit here.

Thanks.

-- 
tejun

Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

[Andrea Righi]

On Fri, Oct 17, 2025 at 05:47:45AM -1000, Tejun Heo wrote:
> On Fri, Oct 17, 2025 at 11:25:53AM +0200, Andrea Righi wrote:
> > +static struct task_struct *
> > +ext_server_pick_task(struct sched_dl_entity *dl_se, struct rq_flags *rf)
> > +{
> > +	return pick_task_scx(dl_se->rq, rf);
> > +}
> 
> I wonder whether we should tell pick_task_scx() to suppress the
> rq_modified_above() test in this case as a fair or RT task being enqueued
> has no reason to restart the picking process. While it will behave fine on
> retry, it's probably useful to be explicit here.

Yeah, that's a valid point. Maybe we can add a new flag to rq->scx.flags?
Something like SCX_RQ_DL_SERVER_PICK?

Thanks,
-Andrea

Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

[Tejun Heo]

On Fri, Oct 17, 2025 at 08:58:35PM +0200, Andrea Righi wrote:
> On Fri, Oct 17, 2025 at 05:47:45AM -1000, Tejun Heo wrote:
> > On Fri, Oct 17, 2025 at 11:25:53AM +0200, Andrea Righi wrote:
> > > +static struct task_struct *
> > > +ext_server_pick_task(struct sched_dl_entity *dl_se, struct rq_flags *rf)
> > > +{
> > > +	return pick_task_scx(dl_se->rq, rf);
> > > +}
> > 
> > I wonder whether we should tell pick_task_scx() to suppress the
> > rq_modified_above() test in this case as a fair or RT task being enqueued
> > has no reason to restart the picking process. While it will behave fine on
> > retry, it's probably useful to be explicit here.
> 
> Yeah, that's a valid point. Maybe we can add a new flag to rq->scx.flags?
> Something like SCX_RQ_DL_SERVER_PICK?

We can factor out the internals of pick_task_scx() into a separate function
and add a flag there?

Thanks.

-- 
tejun

Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

[Andrea Righi]

On Fri, Oct 17, 2025 at 09:04:23AM -1000, Tejun Heo wrote:
> On Fri, Oct 17, 2025 at 08:58:35PM +0200, Andrea Righi wrote:
> > On Fri, Oct 17, 2025 at 05:47:45AM -1000, Tejun Heo wrote:
> > > On Fri, Oct 17, 2025 at 11:25:53AM +0200, Andrea Righi wrote:
> > > > +static struct task_struct *
> > > > +ext_server_pick_task(struct sched_dl_entity *dl_se, struct rq_flags *rf)
> > > > +{
> > > > +	return pick_task_scx(dl_se->rq, rf);
> > > > +}
> > > 
> > > I wonder whether we should tell pick_task_scx() to suppress the
> > > rq_modified_above() test in this case as a fair or RT task being enqueued
> > > has no reason to restart the picking process. While it will behave fine on
> > > retry, it's probably useful to be explicit here.
> > 
> > Yeah, that's a valid point. Maybe we can add a new flag to rq->scx.flags?
> > Something like SCX_RQ_DL_SERVER_PICK?
> 
> We can factor out the internals of pick_task_scx() into a separate function
> and add a flag there?

Much better, I like that. Ok, I'll incorporate this change and send a new
version.

Thanks,
-Andrea

Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

[Juri Lelli]

Hi!

On 17/10/25 11:25, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@nvidia.com>
> 
> sched_ext currently suffers starvation due to RT. The same workload when
> converted to EXT can get zero runtime if RT is 100% running, causing EXT
> processes to stall. Fix it by adding a DL server for EXT.
> 
> A kselftest is also provided later to verify:
> 
> ./runner -t rt_stall
> ===== START =====
> TEST: rt_stall
> DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks
> OUTPUT:
> TAP version 13
> 1..1
> ok 1 PASS: CFS task got more than 4.00% of runtime
> 
> [ arighi: drop ->balance() now that pick_task() has an rf argument ]
> 
> Cc: Luigi De Matteis <ldematteis123@gmail.com>
> Co-developed-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> ---
>  kernel/sched/core.c     |  3 +++
>  kernel/sched/deadline.c |  2 +-
>  kernel/sched/ext.c      | 51 +++++++++++++++++++++++++++++++++++++++--
>  kernel/sched/sched.h    |  2 ++
>  4 files changed, 55 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 096e8d03d85e7..31a9c9381c63f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8679,6 +8679,9 @@ void __init sched_init(void)
>  		hrtick_rq_init(rq);
>  		atomic_set(&rq->nr_iowait, 0);
>  		fair_server_init(rq);
> +#ifdef CONFIG_SCHED_CLASS_EXT
> +		ext_server_init(rq);
> +#endif
>  
>  #ifdef CONFIG_SCHED_CORE
>  		rq->core = rq;
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 0680e0186577a..3c1fd2190949e 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1504,7 +1504,7 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
>  	 * The fair server (sole dl_server) does not account for real-time

Fair server is not alone anymore. :))

Please update the comment as well.

>  	 * workload because it is running fair work.
>  	 */
> -	if (dl_se == &rq->fair_server)
> +	if (dl_se->dl_server)
>  		return;
>  
>  #ifdef CONFIG_RT_GROUP_SCHED

...

> @@ -1487,6 +1499,11 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int deq_flags
>  	sub_nr_running(rq, 1);
>  
>  	dispatch_dequeue(rq, p);
> +
> +	/* Stop the server if this was the last task */
> +	if (rq->scx.nr_running == 0)
> +		dl_server_stop(&rq->ext_server);
> +

Do we want to use the delayed stop behavior for scx-server as we have
for fair-server? Wonder if it's a matter of removing this explicit stop
and wait for a full period to elapse as we do for fair. It should reduce
timer reprogramming overhead for scx as well.

Thanks,
Juri

Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

[Andrea Righi]

Hi Juri,

On Mon, Oct 20, 2025 at 01:58:50PM +0200, Juri Lelli wrote:
> Hi!
> 
> On 17/10/25 11:25, Andrea Righi wrote:
> > From: Joel Fernandes <joelagnelf@nvidia.com>
> > 
> > sched_ext currently suffers starvation due to RT. The same workload when
> > converted to EXT can get zero runtime if RT is 100% running, causing EXT
> > processes to stall. Fix it by adding a DL server for EXT.
> > 
> > A kselftest is also provided later to verify:
> > 
> > ./runner -t rt_stall
> > ===== START =====
> > TEST: rt_stall
> > DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks
> > OUTPUT:
> > TAP version 13
> > 1..1
> > ok 1 PASS: CFS task got more than 4.00% of runtime
> > 
> > [ arighi: drop ->balance() now that pick_task() has an rf argument ]
> > 
> > Cc: Luigi De Matteis <ldematteis123@gmail.com>
> > Co-developed-by: Andrea Righi <arighi@nvidia.com>
> > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > ---
> >  kernel/sched/core.c     |  3 +++
> >  kernel/sched/deadline.c |  2 +-
> >  kernel/sched/ext.c      | 51 +++++++++++++++++++++++++++++++++++++++--
> >  kernel/sched/sched.h    |  2 ++
> >  4 files changed, 55 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 096e8d03d85e7..31a9c9381c63f 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -8679,6 +8679,9 @@ void __init sched_init(void)
> >  		hrtick_rq_init(rq);
> >  		atomic_set(&rq->nr_iowait, 0);
> >  		fair_server_init(rq);
> > +#ifdef CONFIG_SCHED_CLASS_EXT
> > +		ext_server_init(rq);
> > +#endif
> >  
> >  #ifdef CONFIG_SCHED_CORE
> >  		rq->core = rq;
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 0680e0186577a..3c1fd2190949e 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1504,7 +1504,7 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
> >  	 * The fair server (sole dl_server) does not account for real-time
> 
> Fair server is not alone anymore. :))
> 
> Please update the comment as well.
> 
> >  	 * workload because it is running fair work.
> >  	 */
> > -	if (dl_se == &rq->fair_server)
> > +	if (dl_se->dl_server)
> >  		return;
> >  
> >  #ifdef CONFIG_RT_GROUP_SCHED
> 
> ...
> 
> > @@ -1487,6 +1499,11 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int deq_flags
> >  	sub_nr_running(rq, 1);
> >  
> >  	dispatch_dequeue(rq, p);
> > +
> > +	/* Stop the server if this was the last task */
> > +	if (rq->scx.nr_running == 0)
> > +		dl_server_stop(&rq->ext_server);
> > +
> 
> Do we want to use the delayed stop behavior for scx-server as we have
> for fair-server? Wonder if it's a matter of removing this explicit stop
> and wait for a full period to elapse as we do for fair. It should reduce
> timer reprogramming overhead for scx as well.

So, IIUC we could just remove this explicit dl_server_stop() and the server
would naturally stop at the end of its current deadline period, if there
are still no runnable tasks, right?

In that case it's worth a try.

Thanks,
-Andrea

Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

[Juri Lelli]

On 20/10/25 15:50, Andrea Righi wrote:
> Hi Juri,
> 
> On Mon, Oct 20, 2025 at 01:58:50PM +0200, Juri Lelli wrote:
> > Hi!
> > 
> > On 17/10/25 11:25, Andrea Righi wrote:

...

> > > @@ -1487,6 +1499,11 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int deq_flags
> > >  	sub_nr_running(rq, 1);
> > >  
> > >  	dispatch_dequeue(rq, p);
> > > +
> > > +	/* Stop the server if this was the last task */
> > > +	if (rq->scx.nr_running == 0)
> > > +		dl_server_stop(&rq->ext_server);
> > > +
> > 
> > Do we want to use the delayed stop behavior for scx-server as we have
> > for fair-server? Wonder if it's a matter of removing this explicit stop
> > and wait for a full period to elapse as we do for fair. It should reduce
> > timer reprogramming overhead for scx as well.
> 
> So, IIUC we could just remove this explicit dl_server_stop() and the server
> would naturally stop at the end of its current deadline period, if there
> are still no runnable tasks, right?

Right, that is what I'd expect. But this part tricked me several times
already, so I am not 100% certain (Peter please keep me honest :).

> In that case it's worth a try.

[PATCH 07/14] sched/debug: Add support to change sched_ext server params

[Andrea Righi]

From: Joel Fernandes <joelagnelf@nvidia.com>

When a sched_ext server is loaded, tasks in CFS are converted to run in
sched_ext class. Add support to modify the ext server parameters similar
to how the fair server parameters are modified.

Re-use common code between ext and fair servers as needed.

[ arighi: Use dl_se->dl_server to determine if dl_se is a DL server, as
          suggested by PeterZ. ]

Co-developed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/sched/debug.c | 149 ++++++++++++++++++++++++++++++++++++-------
 1 file changed, 125 insertions(+), 24 deletions(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index e71f6618c1a6a..00ad35b812f76 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -336,14 +336,16 @@ enum dl_param {
 	DL_PERIOD,
 };
 
-static unsigned long fair_server_period_max = (1UL << 22) * NSEC_PER_USEC; /* ~4 seconds */
-static unsigned long fair_server_period_min = (100) * NSEC_PER_USEC;     /* 100 us */
+static unsigned long dl_server_period_max = (1UL << 22) * NSEC_PER_USEC; /* ~4 seconds */
+static unsigned long dl_server_period_min = (100) * NSEC_PER_USEC;     /* 100 us */
 
-static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubuf,
-				       size_t cnt, loff_t *ppos, enum dl_param param)
+static ssize_t sched_server_write_common(struct file *filp, const char __user *ubuf,
+					 size_t cnt, loff_t *ppos, enum dl_param param,
+					 void *server)
 {
 	long cpu = (long) ((struct seq_file *) filp->private_data)->private;
 	struct rq *rq = cpu_rq(cpu);
+	struct sched_dl_entity *dl_se = (struct sched_dl_entity *)server;
 	u64 runtime, period;
 	int retval = 0;
 	size_t err;
@@ -356,8 +358,8 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
 	scoped_guard (rq_lock_irqsave, rq) {
 		bool is_active;
 
-		runtime  = rq->fair_server.dl_runtime;
-		period = rq->fair_server.dl_period;
+		runtime = dl_se->dl_runtime;
+		period = dl_se->dl_period;
 
 		switch (param) {
 		case DL_RUNTIME:
@@ -373,25 +375,25 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
 		}
 
 		if (runtime > period ||
-		    period > fair_server_period_max ||
-		    period < fair_server_period_min) {
+		    period > dl_server_period_max ||
+		    period < dl_server_period_min) {
 			return  -EINVAL;
 		}
 
-		is_active = dl_server_active(&rq->fair_server);
+		is_active = dl_server_active(dl_se);
 		if (is_active) {
 			update_rq_clock(rq);
-			dl_server_stop(&rq->fair_server);
+			dl_server_stop(dl_se);
 		}
 
-		retval = dl_server_apply_params(&rq->fair_server, runtime, period, 0);
+		retval = dl_server_apply_params(dl_se, runtime, period, 0);
 
 		if (!runtime)
-			printk_deferred("Fair server disabled in CPU %d, system may crash due to starvation.\n",
-					cpu_of(rq));
+			printk_deferred("%s server disabled on CPU %d, system may crash due to starvation.\n",
+					server == &rq->fair_server ? "Fair" : "Ext", cpu_of(rq));
 
 		if (is_active)
-			dl_server_start(&rq->fair_server);
+			dl_server_start(dl_se);
 
 		if (retval < 0)
 			return retval;
@@ -401,36 +403,42 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
 	return cnt;
 }
 
-static size_t sched_fair_server_show(struct seq_file *m, void *v, enum dl_param param)
+static size_t sched_server_show_common(struct seq_file *m, void *v, enum dl_param param,
+				       void *server)
 {
-	unsigned long cpu = (unsigned long) m->private;
-	struct rq *rq = cpu_rq(cpu);
+	struct sched_dl_entity *dl_se = (struct sched_dl_entity *)server;
 	u64 value;
 
 	switch (param) {
 	case DL_RUNTIME:
-		value = rq->fair_server.dl_runtime;
+		value = dl_se->dl_runtime;
 		break;
 	case DL_PERIOD:
-		value = rq->fair_server.dl_period;
+		value = dl_se->dl_period;
 		break;
 	}
 
 	seq_printf(m, "%llu\n", value);
 	return 0;
-
 }
 
 static ssize_t
 sched_fair_server_runtime_write(struct file *filp, const char __user *ubuf,
 				size_t cnt, loff_t *ppos)
 {
-	return sched_fair_server_write(filp, ubuf, cnt, ppos, DL_RUNTIME);
+	long cpu = (long) ((struct seq_file *) filp->private_data)->private;
+	struct rq *rq = cpu_rq(cpu);
+
+	return sched_server_write_common(filp, ubuf, cnt, ppos, DL_RUNTIME,
+					&rq->fair_server);
 }
 
 static int sched_fair_server_runtime_show(struct seq_file *m, void *v)
 {
-	return sched_fair_server_show(m, v, DL_RUNTIME);
+	unsigned long cpu = (unsigned long) m->private;
+	struct rq *rq = cpu_rq(cpu);
+
+	return sched_server_show_common(m, v, DL_RUNTIME, &rq->fair_server);
 }
 
 static int sched_fair_server_runtime_open(struct inode *inode, struct file *filp)
@@ -446,16 +454,55 @@ static const struct file_operations fair_server_runtime_fops = {
 	.release	= single_release,
 };
 
+static ssize_t
+sched_ext_server_runtime_write(struct file *filp, const char __user *ubuf,
+			       size_t cnt, loff_t *ppos)
+{
+	long cpu = (long) ((struct seq_file *) filp->private_data)->private;
+	struct rq *rq = cpu_rq(cpu);
+
+	return sched_server_write_common(filp, ubuf, cnt, ppos, DL_RUNTIME,
+					&rq->ext_server);
+}
+
+static int sched_ext_server_runtime_show(struct seq_file *m, void *v)
+{
+	unsigned long cpu = (unsigned long) m->private;
+	struct rq *rq = cpu_rq(cpu);
+
+	return sched_server_show_common(m, v, DL_RUNTIME, &rq->ext_server);
+}
+
+static int sched_ext_server_runtime_open(struct inode *inode, struct file *filp)
+{
+	return single_open(filp, sched_ext_server_runtime_show, inode->i_private);
+}
+
+static const struct file_operations ext_server_runtime_fops = {
+	.open		= sched_ext_server_runtime_open,
+	.write		= sched_ext_server_runtime_write,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
 static ssize_t
 sched_fair_server_period_write(struct file *filp, const char __user *ubuf,
 			       size_t cnt, loff_t *ppos)
 {
-	return sched_fair_server_write(filp, ubuf, cnt, ppos, DL_PERIOD);
+	long cpu = (long) ((struct seq_file *) filp->private_data)->private;
+	struct rq *rq = cpu_rq(cpu);
+
+	return sched_server_write_common(filp, ubuf, cnt, ppos, DL_PERIOD,
+					&rq->fair_server);
 }
 
 static int sched_fair_server_period_show(struct seq_file *m, void *v)
 {
-	return sched_fair_server_show(m, v, DL_PERIOD);
+	unsigned long cpu = (unsigned long) m->private;
+	struct rq *rq = cpu_rq(cpu);
+
+	return sched_server_show_common(m, v, DL_PERIOD, &rq->fair_server);
 }
 
 static int sched_fair_server_period_open(struct inode *inode, struct file *filp)
@@ -471,6 +518,38 @@ static const struct file_operations fair_server_period_fops = {
 	.release	= single_release,
 };
 
+static ssize_t
+sched_ext_server_period_write(struct file *filp, const char __user *ubuf,
+			      size_t cnt, loff_t *ppos)
+{
+	long cpu = (long) ((struct seq_file *) filp->private_data)->private;
+	struct rq *rq = cpu_rq(cpu);
+
+	return sched_server_write_common(filp, ubuf, cnt, ppos, DL_PERIOD,
+					&rq->ext_server);
+}
+
+static int sched_ext_server_period_show(struct seq_file *m, void *v)
+{
+	unsigned long cpu = (unsigned long) m->private;
+	struct rq *rq = cpu_rq(cpu);
+
+	return sched_server_show_common(m, v, DL_PERIOD, &rq->ext_server);
+}
+
+static int sched_ext_server_period_open(struct inode *inode, struct file *filp)
+{
+	return single_open(filp, sched_ext_server_period_show, inode->i_private);
+}
+
+static const struct file_operations ext_server_period_fops = {
+	.open		= sched_ext_server_period_open,
+	.write		= sched_ext_server_period_write,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
 static struct dentry *debugfs_sched;
 
 static void debugfs_fair_server_init(void)
@@ -494,6 +573,27 @@ static void debugfs_fair_server_init(void)
 	}
 }
 
+static void debugfs_ext_server_init(void)
+{
+	struct dentry *d_ext;
+	unsigned long cpu;
+
+	d_ext = debugfs_create_dir("ext_server", debugfs_sched);
+	if (!d_ext)
+		return;
+
+	for_each_possible_cpu(cpu) {
+		struct dentry *d_cpu;
+		char buf[32];
+
+		snprintf(buf, sizeof(buf), "cpu%lu", cpu);
+		d_cpu = debugfs_create_dir(buf, d_ext);
+
+		debugfs_create_file("runtime", 0644, d_cpu, (void *) cpu, &ext_server_runtime_fops);
+		debugfs_create_file("period", 0644, d_cpu, (void *) cpu, &ext_server_period_fops);
+	}
+}
+
 static __init int sched_init_debug(void)
 {
 	struct dentry __maybe_unused *numa;
@@ -532,6 +632,7 @@ static __init int sched_init_debug(void)
 	debugfs_create_file("debug", 0444, debugfs_sched, NULL, &sched_debug_fops);
 
 	debugfs_fair_server_init();
+	debugfs_ext_server_init();
 
 	return 0;
 }
-- 
2.51.0

Re: [PATCH 07/14] sched/debug: Add support to change sched_ext server params

[Juri Lelli]

Hi!

On 17/10/25 11:25, Andrea Righi wrote:
> From: Joel Fernandes <joelagnelf@nvidia.com>
> 
> When a sched_ext server is loaded, tasks in CFS are converted to run in
> sched_ext class. Add support to modify the ext server parameters similar
> to how the fair server parameters are modified.
> 
> Re-use common code between ext and fair servers as needed.
> 
> [ arighi: Use dl_se->dl_server to determine if dl_se is a DL server, as
>           suggested by PeterZ. ]
> 
> Co-developed-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> ---

...

> @@ -373,25 +375,25 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
>  		}
>  
>  		if (runtime > period ||
> -		    period > fair_server_period_max ||
> -		    period < fair_server_period_min) {
> +		    period > dl_server_period_max ||
> +		    period < dl_server_period_min) {
>  			return  -EINVAL;
>  		}
>  
> -		is_active = dl_server_active(&rq->fair_server);
> +		is_active = dl_server_active(dl_se);
>  		if (is_active) {
>  			update_rq_clock(rq);
> -			dl_server_stop(&rq->fair_server);
> +			dl_server_stop(dl_se);
>  		}
>  
> -		retval = dl_server_apply_params(&rq->fair_server, runtime, period, 0);
> +		retval = dl_server_apply_params(dl_se, runtime, period, 0);
>  
>  		if (!runtime)
> -			printk_deferred("Fair server disabled in CPU %d, system may crash due to starvation.\n",
> -					cpu_of(rq));
> +			printk_deferred("%s server disabled on CPU %d, system may crash due to starvation.\n",
> +					server == &rq->fair_server ? "Fair" : "Ext", cpu_of(rq));

Guess this might get convoluted if are ever going to add an additional
dl-server, but I fail to see that happening atm (to service what?).

Reviewed-by: Juri Lelli <juri.lelli@redhat.com>

Thanks,
Juri

Re: [PATCH 07/14] sched/debug: Add support to change sched_ext server params

[Andrea Righi]

On Mon, Oct 20, 2025 at 02:45:50PM +0200, Juri Lelli wrote:
> Hi!
> 
> On 17/10/25 11:25, Andrea Righi wrote:
> > From: Joel Fernandes <joelagnelf@nvidia.com>
> > 
> > When a sched_ext server is loaded, tasks in CFS are converted to run in
> > sched_ext class. Add support to modify the ext server parameters similar
> > to how the fair server parameters are modified.
> > 
> > Re-use common code between ext and fair servers as needed.
> > 
> > [ arighi: Use dl_se->dl_server to determine if dl_se is a DL server, as
> >           suggested by PeterZ. ]
> > 
> > Co-developed-by: Andrea Righi <arighi@nvidia.com>
> > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > ---
> 
> ...
> 
> > @@ -373,25 +375,25 @@ static ssize_t sched_fair_server_write(struct file *filp, const char __user *ubu
> >  		}
> >  
> >  		if (runtime > period ||
> > -		    period > fair_server_period_max ||
> > -		    period < fair_server_period_min) {
> > +		    period > dl_server_period_max ||
> > +		    period < dl_server_period_min) {
> >  			return  -EINVAL;
> >  		}
> >  
> > -		is_active = dl_server_active(&rq->fair_server);
> > +		is_active = dl_server_active(dl_se);
> >  		if (is_active) {
> >  			update_rq_clock(rq);
> > -			dl_server_stop(&rq->fair_server);
> > +			dl_server_stop(dl_se);
> >  		}
> >  
> > -		retval = dl_server_apply_params(&rq->fair_server, runtime, period, 0);
> > +		retval = dl_server_apply_params(dl_se, runtime, period, 0);
> >  
> >  		if (!runtime)
> > -			printk_deferred("Fair server disabled in CPU %d, system may crash due to starvation.\n",
> > -					cpu_of(rq));
> > +			printk_deferred("%s server disabled on CPU %d, system may crash due to starvation.\n",
> > +					server == &rq->fair_server ? "Fair" : "Ext", cpu_of(rq));
> 
> Guess this might get convoluted if are ever going to add an additional
> dl-server, but I fail to see that happening atm (to service what?).

We could add a ->server_class() method that returns the name or something
similar, but it's probably a bit overkill, since we have just two dl
servers at the moment (and I don't see any use case to have more...).

Thanks,
-Andrea

> 
> Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
> 
> Thanks,
> Juri
> 

[PATCH 08/14] sched/deadline: Add support to remove DL server's bandwidth contribution

[Andrea Righi]

During switching from sched_ext to FAIR tasks and vice-versa, we need
support for removing the bandwidth contribution of either DL server. Add
support for the same.

Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/deadline.c | 31 +++++++++++++++++++++++++++++++
 kernel/sched/sched.h    |  1 +
 2 files changed, 32 insertions(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 3c1fd2190949e..d585be4014427 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1684,6 +1684,12 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
 		dl_rq_change_utilization(rq, dl_se, new_bw);
 	}
 
+	/* Clear these so that the dl_server is reinitialized */
+	if (new_bw == 0) {
+		dl_se->dl_defer = 0;
+		dl_se->dl_server = 0;
+	}
+
 	dl_se->dl_runtime = runtime;
 	dl_se->dl_deadline = period;
 	dl_se->dl_period = period;
@@ -1697,6 +1703,31 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
 	return retval;
 }
 
+/**
+ * dl_server_remove_params - Remove bandwidth reservation for a DL server
+ * @dl_se: The DL server entity to remove bandwidth for
+ *
+ * This function removes the bandwidth reservation for a DL server entity,
+ * cleaning up all bandwidth accounting and server state.
+ *
+ * Returns: 0 on success, negative error code on failure
+ */
+int dl_server_remove_params(struct sched_dl_entity *dl_se)
+{
+	if (!dl_se->dl_server)
+		return 0; /* Already disabled */
+
+	/*
+	 * First dequeue if still queued. It should not be queued since
+	 * we call this only after the last dl_server_stop().
+	 */
+	if (WARN_ON_ONCE(on_dl_rq(dl_se)))
+		dequeue_dl_entity(dl_se, DEQUEUE_SLEEP);
+
+	/* Remove bandwidth reservation */
+	return dl_server_apply_params(dl_se, 0, dl_se->dl_period, false);
+}
+
 /*
  * Update the current task's runtime statistics (provided it is still
  * a -deadline task and has not been removed from the dl_rq).
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 55f8fbb306517..2c1404e961171 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -419,6 +419,7 @@ extern void ext_server_init(struct rq *rq);
 extern void __dl_server_attach_root(struct sched_dl_entity *dl_se, struct rq *rq);
 extern int dl_server_apply_params(struct sched_dl_entity *dl_se,
 		    u64 runtime, u64 period, bool init);
+extern int dl_server_remove_params(struct sched_dl_entity *dl_se);
 
 static inline bool dl_server_active(struct sched_dl_entity *dl_se)
 {
-- 
2.51.0

Re: [PATCH 08/14] sched/deadline: Add support to remove DL server's bandwidth contribution

[Juri Lelli]

Hi!

On 17/10/25 11:25, Andrea Righi wrote:
> During switching from sched_ext to FAIR tasks and vice-versa, we need
> support for removing the bandwidth contribution of either DL server. Add
> support for the same.
> 
> Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/deadline.c | 31 +++++++++++++++++++++++++++++++
>  kernel/sched/sched.h    |  1 +
>  2 files changed, 32 insertions(+)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 3c1fd2190949e..d585be4014427 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1684,6 +1684,12 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
>  		dl_rq_change_utilization(rq, dl_se, new_bw);
>  	}
>  
> +	/* Clear these so that the dl_server is reinitialized */
> +	if (new_bw == 0) {
> +		dl_se->dl_defer = 0;
> +		dl_se->dl_server = 0;
> +	}
> +
>  	dl_se->dl_runtime = runtime;
>  	dl_se->dl_deadline = period;
>  	dl_se->dl_period = period;
> @@ -1697,6 +1703,31 @@ int dl_server_apply_params(struct sched_dl_entity *dl_se, u64 runtime, u64 perio
>  	return retval;
>  }
>  
> +/**
> + * dl_server_remove_params - Remove bandwidth reservation for a DL server
> + * @dl_se: The DL server entity to remove bandwidth for
> + *
> + * This function removes the bandwidth reservation for a DL server entity,
> + * cleaning up all bandwidth accounting and server state.
> + *
> + * Returns: 0 on success, negative error code on failure
> + */
> +int dl_server_remove_params(struct sched_dl_entity *dl_se)
> +{
> +	if (!dl_se->dl_server)
> +		return 0; /* Already disabled */
> +
> +	/*
> +	 * First dequeue if still queued. It should not be queued since
> +	 * we call this only after the last dl_server_stop().
> +	 */
> +	if (WARN_ON_ONCE(on_dl_rq(dl_se)))
> +		dequeue_dl_entity(dl_se, DEQUEUE_SLEEP);
> +
> +	/* Remove bandwidth reservation */
> +	return dl_server_apply_params(dl_se, 0, dl_se->dl_period, false);
> +}

I am not sure this is correct wrt inactive_task_timer and
task_non_contending. I fear that removing bw immediately might break
other deadline entities guarantees (especially if one then maliciously
add/removes dl-servers quickly). I kind of think (but again not sure,
please Peter and other keep me honest :) we should be waiting for
inactive_task_timer to fire (if stopping before 0-lag) and let it clean
things up at that point (like we do for simple tasks).

You seem to have additional fixes later on in the series that might be
caused by what I describe above.

Thinking more about this I actually wonder if we need this (well it's
coming up with later patches) mechanism for automatically removing
servers based on fair vs. scx state (full/partial). If we are going to
manage dl-servers bw explicitly and separately [1], maybe we can just
leave the burden to the user (or middleware) of doing that via the
configuration interface?

Thanks,
Juri

1 - https://lore.kernel.org/lkml/aPYDhjqe99F91FTW@jlelli-thinkpadt14gen4.remote.csb/ 

[PATCH 09/14] sched/deadline: Account ext server bandwidth

[Andrea Righi]

Always account for both the ext_server and fair_server bandwidths,
especially during CPU hotplug operations. Ignoring either can lead to
imbalances in total_bw when sched_ext schedulers are active and CPUs are
brought online / offline.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/deadline.c | 29 +++++++++++++++++++++--------
 kernel/sched/topology.c |  5 +++++
 2 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index d585be4014427..ba2d58bfc82c8 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2984,9 +2984,17 @@ void dl_clear_root_domain(struct root_domain *rd)
 	 * them, we need to account for them here explicitly.
 	 */
 	for_each_cpu(i, rd->span) {
-		struct sched_dl_entity *dl_se = &cpu_rq(i)->fair_server;
+		struct sched_dl_entity *dl_se;
 
-		if (dl_server(dl_se) && cpu_active(i))
+		if (!cpu_active(i))
+			continue;
+
+		dl_se = &cpu_rq(i)->fair_server;
+		if (dl_server(dl_se))
+			__dl_add(&rd->dl_bw, dl_se->dl_bw, dl_bw_cpus(i));
+
+		dl_se = &cpu_rq(i)->ext_server;
+		if (dl_server(dl_se))
 			__dl_add(&rd->dl_bw, dl_se->dl_bw, dl_bw_cpus(i));
 	}
 }
@@ -3485,6 +3493,7 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
 	struct dl_bw *dl_b;
 	bool overflow = 0;
 	u64 fair_server_bw = 0;
+	u64 ext_server_bw = 0;
 
 	rcu_read_lock_sched();
 	dl_b = dl_bw_of(cpu);
@@ -3517,27 +3526,31 @@ static int dl_bw_manage(enum dl_bw_request req, int cpu, u64 dl_bw)
 		cap -= arch_scale_cpu_capacity(cpu);
 
 		/*
-		 * cpu is going offline and NORMAL tasks will be moved away
-		 * from it. We can thus discount dl_server bandwidth
-		 * contribution as it won't need to be servicing tasks after
-		 * the cpu is off.
+		 * cpu is going offline and NORMAL and EXT tasks will be
+		 * moved away from it. We can thus discount dl_server
+		 * bandwidth contribution as it won't need to be servicing
+		 * tasks after the cpu is off.
 		 */
 		if (cpu_rq(cpu)->fair_server.dl_server)
 			fair_server_bw = cpu_rq(cpu)->fair_server.dl_bw;
 
+		if (cpu_rq(cpu)->ext_server.dl_server)
+			ext_server_bw = cpu_rq(cpu)->ext_server.dl_bw;
+
 		/*
 		 * Not much to check if no DEADLINE bandwidth is present.
 		 * dl_servers we can discount, as tasks will be moved out the
 		 * offlined CPUs anyway.
 		 */
-		if (dl_b->total_bw - fair_server_bw > 0) {
+		if (dl_b->total_bw - fair_server_bw - ext_server_bw > 0) {
 			/*
 			 * Leaving at least one CPU for DEADLINE tasks seems a
 			 * wise thing to do. As said above, cpu is not offline
 			 * yet, so account for that.
 			 */
 			if (dl_bw_cpus(cpu) - 1)
-				overflow = __dl_overflow(dl_b, cap, fair_server_bw, 0);
+				overflow = __dl_overflow(dl_b, cap,
+							 fair_server_bw + ext_server_bw, 0);
 			else
 				overflow = 1;
 		}
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 711076aa49801..1ec8e74b80219 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -508,6 +508,11 @@ void rq_attach_root(struct rq *rq, struct root_domain *rd)
 	if (rq->fair_server.dl_server)
 		__dl_server_attach_root(&rq->fair_server, rq);
 
+#ifdef CONFIG_SCHED_CLASS_EXT
+	if (rq->ext_server.dl_server)
+		__dl_server_attach_root(&rq->ext_server, rq);
+#endif
+
 	rq_unlock_irqrestore(rq, &rf);
 
 	if (old_rd)
-- 
2.51.0

[PATCH 10/14] sched/deadline: Allow to initialize DL server when needed

[Andrea Righi]

When switching between fair and sched_ext, we need to initialize the
bandwidth contribution of the DL server independently for each class.

Add support for on-demand initialization to handle such transitions.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/deadline.c | 36 +++++++++++++++++++++++++++++-------
 kernel/sched/sched.h    |  1 +
 2 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index ba2d58bfc82c8..16e229180bf46 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1576,6 +1576,32 @@ void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
 		update_curr_dl_se(dl_se->rq, dl_se, delta_exec);
 }
 
+/**
+ * dl_server_init_params - Initialize bandwidth reservation for a DL server
+ * @dl_se: The DL server entity to remove bandwidth for
+ *
+ * This function initializes the bandwidth reservation for a DL server
+ * entity, its bandwidth accounting and server state.
+ *
+ * Returns: 0 on success, negative error code on failure
+ */
+int dl_server_init_params(struct sched_dl_entity *dl_se)
+{
+	u64 runtime =  50 * NSEC_PER_MSEC;
+	u64 period = 1000 * NSEC_PER_MSEC;
+	int err;
+
+	err = dl_server_apply_params(dl_se, runtime, period, 1);
+	if (err)
+		return err;
+
+	dl_se->dl_server = 1;
+	dl_se->dl_defer = 1;
+	setup_new_dl_entity(dl_se);
+
+	return err;
+}
+
 void dl_server_start(struct sched_dl_entity *dl_se)
 {
 	struct rq *rq = dl_se->rq;
@@ -1615,8 +1641,7 @@ void sched_init_dl_servers(void)
 	struct sched_dl_entity *dl_se;
 
 	for_each_online_cpu(cpu) {
-		u64 runtime =  50 * NSEC_PER_MSEC;
-		u64 period = 1000 * NSEC_PER_MSEC;
+		int err;
 
 		rq = cpu_rq(cpu);
 
@@ -1626,11 +1651,8 @@ void sched_init_dl_servers(void)
 
 		WARN_ON(dl_server(dl_se));
 
-		dl_server_apply_params(dl_se, runtime, period, 1);
-
-		dl_se->dl_server = 1;
-		dl_se->dl_defer = 1;
-		setup_new_dl_entity(dl_se);
+		err = dl_server_init_params(dl_se);
+		WARN_ON_ONCE(err);
 	}
 }
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 2c1404e961171..eda1141f94fd5 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -419,6 +419,7 @@ extern void ext_server_init(struct rq *rq);
 extern void __dl_server_attach_root(struct sched_dl_entity *dl_se, struct rq *rq);
 extern int dl_server_apply_params(struct sched_dl_entity *dl_se,
 		    u64 runtime, u64 period, bool init);
+extern int dl_server_init_params(struct sched_dl_entity *dl_se);
 extern int dl_server_remove_params(struct sched_dl_entity *dl_se);
 
 static inline bool dl_server_active(struct sched_dl_entity *dl_se)
-- 
2.51.0

[PATCH 11/14] sched/deadline: Fix DL server crash in inactive_timer callback

[Andrea Righi]

From: Joel Fernandes <joelagnelf@nvidia.com>

When sched_ext is rapidly disabled/enabled (the reload_loop selftest),
the following crash is observed. This happens because the timer handler
could not be cancelled and still fires even though the dl_server
bandwidth may have been removed via dl_server_remove_params().
hrtimer_try_to_cancel() does not guarantee timer cancellation. This
results in a NULL pointer dereference as 'p' is bogus for a dl_se.

This happens because the timer may be about to run, but its softirq has
not executed yet. Because of that hrtimer_try_to_cancel() cannot prevent
the timer from being canceled, however dl_server is still set to NULL by
dl_server_apply_params(). When the timer handler eventually runs, it
crashes.

[   24.771835] BUG: kernel NULL pointer dereference, address: 000000000000006c
[   24.772097] #PF: supervisor read access in kernel mode
[   24.772248] #PF: error_code(0x0000) - not-present page
[   24.772404] PGD 0 P4D 0
[   24.772499] Oops: Oops: 0000 [#1] SMP PTI
[   24.772614] CPU: 9 UID: 0 PID: 0 Comm: swapper/9 [..] #74 PREEMPT(voluntary)
[   24.772932] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), [...]
[   24.773149] Sched_ext: maximal (disabling)
[   24.773944] RSP: 0018:ffffb162c0348ee0 EFLAGS: 00010046
[   24.774100] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff88d4412f1800
[   24.774302] RDX: 0000000000000001 RSI: 0000000000000010 RDI: ffffffffac939240
[   24.774498] RBP: ffff88d47e65b940 R08: 0000000000000010 R09: 00000008bad3370a
[   24.774742] R10: 0000000000000000 R11: ffffffffa9f159d0 R12: ffff88d47e65b900
[   24.774962] R13: ffff88d47e65b960 R14: ffff88d47e66a340 R15: ffff88d47e66aed0
[   24.775182] FS:  0000000000000000(0000) GS:ffff88d4d1d56000(0000) knlGS:[...]
[   24.775392] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   24.775579] CR2: 000000000000006c CR3: 0000000002bb0003 CR4: 0000000000770ef0
[   24.775810] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   24.776023] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   24.776225] PKRU: 55555554
[   24.776292] Call Trace:
[   24.776373]  <IRQ>
[   24.776453]  ? __pfx_inactive_task_timer+0x10/0x10
[   24.776591]  __hrtimer_run_queues+0xf1/0x270
[   24.776744]  hrtimer_interrupt+0xfa/0x220
[   24.776847]  __sysvec_apic_timer_interrupt+0x4d/0x190
[   24.776988]  sysvec_apic_timer_interrupt+0x69/0x80
[   24.777132]  </IRQ>
[   24.777194]  <TASK>
[   24.777256]  asm_sysvec_apic_timer_interrupt+0x1a/0x20

Fix by also checking the DL server's pick_task pointer which only exists
for server tasks. This avoids dereferencing invalid task pointers when
the timer fires after the DL server has been disabled.

[ arighi: replace ->server_has_tasks with ->server_pick_task  ]

Co-developed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/sched/deadline.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 16e229180bf46..7889e95d3309c 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1784,7 +1784,16 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 	struct rq_flags rf;
 	struct rq *rq;
 
-	if (!dl_server(dl_se)) {
+	/*
+	 * It is possible that after dl_server_apply_params(), the
+	 * dl_se->dl_server == NULL, but the inactive timer is still queued
+	 * and could not get canceled.
+	 *
+	 * Double check by looking at ->server_pick_tasks to make sure
+	 * we're dealing with a non-server entity. Otherwise p may be bogus
+	 * and we'll crash.
+	 */
+	if (!dl_server(dl_se) && !dl_se->server_pick_task) {
 		p = dl_task_of(dl_se);
 		rq = task_rq_lock(p, &rf);
 	} else {
@@ -1795,7 +1804,7 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 	sched_clock_tick();
 	update_rq_clock(rq);
 
-	if (dl_server(dl_se))
+	if (dl_server(dl_se) || dl_se->server_pick_task)
 		goto no_task;
 
 	if (!dl_task(p) || READ_ONCE(p->__state) == TASK_DEAD) {
@@ -1823,7 +1832,7 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 	dl_se->dl_non_contending = 0;
 unlock:
 
-	if (!dl_server(dl_se)) {
+	if (!dl_server(dl_se) && !dl_se->server_pick_task) {
 		task_rq_unlock(rq, p, &rf);
 		put_task_struct(p);
 	} else {
-- 
2.51.0

[PATCH 12/14] sched_ext: Selectively enable ext and fair DL servers

[Andrea Righi]

Enable or disable the appropriate DL servers (ext and fair) depending on
whether an scx scheduler is started in full or partial mode:

 - in full mode, disable the fair DL server and enable the ext DL server
   on all online CPUs,
 - in partial mode (%SCX_OPS_SWITCH_PARTIAL), keep both fair and ext DL
   servers active to support tasks in both scheduling classes.

Additionally, handle CPU hotplug events by selectively enabling or
disabling the relevant DL servers on the CPU that is going
offline/online. This ensures correct bandwidth reservation also when
CPUs are brought online or offline.

Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/ext.c | 97 +++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 87 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index bc2aaa3236fd4..c5f3c39972b6b 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2545,6 +2545,57 @@ static void set_cpus_allowed_scx(struct task_struct *p,
 				 p, (struct cpumask *)p->cpus_ptr);
 }
 
+static void dl_server_on(struct rq *rq, bool switch_all)
+{
+	struct rq_flags rf;
+	int err;
+
+	rq_lock_irqsave(rq, &rf);
+	update_rq_clock(rq);
+
+	if (switch_all) {
+		/*
+		 * If all fair tasks are moved to the scx scheduler, we
+		 * don't need the fair DL servers anymore, so remove it.
+		 *
+		 * When the current scx scheduler is unloaded, the fair DL
+		 * server will be re-initialized.
+		 */
+		if (dl_server_active(&rq->fair_server))
+			dl_server_stop(&rq->fair_server);
+		dl_server_remove_params(&rq->fair_server);
+	}
+
+	err = dl_server_init_params(&rq->ext_server);
+	WARN_ON_ONCE(err);
+
+	rq_unlock_irqrestore(rq, &rf);
+}
+
+static void dl_server_off(struct rq *rq, bool switch_all)
+{
+	struct rq_flags rf;
+	int err;
+
+	rq_lock_irqsave(rq, &rf);
+	update_rq_clock(rq);
+
+	if (dl_server_active(&rq->ext_server))
+		dl_server_stop(&rq->ext_server);
+	dl_server_remove_params(&rq->ext_server);
+
+	if (switch_all) {
+		/*
+		 * Re-initialize the fair DL server if it was previously disabled
+		 * because all fair tasks had been moved to the ext class.
+		 */
+		err = dl_server_init_params(&rq->fair_server);
+		WARN_ON_ONCE(err);
+	}
+
+	rq_unlock_irqrestore(rq, &rf);
+}
+
 static void handle_hotplug(struct rq *rq, bool online)
 {
 	struct scx_sched *sch = scx_root;
@@ -2560,9 +2611,20 @@ static void handle_hotplug(struct rq *rq, bool online)
 	if (unlikely(!sch))
 		return;
 
-	if (scx_enabled())
+	if (scx_enabled()) {
+		bool is_switching_all = READ_ONCE(scx_switching_all);
+
 		scx_idle_update_selcpu_topology(&sch->ops);
 
+		/*
+		 * Update ext and fair DL servers on hotplug events.
+		 */
+		if (online)
+			dl_server_on(rq, is_switching_all);
+		else
+			dl_server_off(rq, is_switching_all);
+	}
+
 	if (online && SCX_HAS_OP(sch, cpu_online))
 		SCX_CALL_OP(sch, SCX_KF_UNLOCKED, cpu_online, NULL, cpu);
 	else if (!online && SCX_HAS_OP(sch, cpu_offline))
@@ -3921,6 +3983,7 @@ static void scx_disable_workfn(struct kthread_work *work)
 	struct scx_exit_info *ei = sch->exit_info;
 	struct scx_task_iter sti;
 	struct task_struct *p;
+	bool is_switching_all = READ_ONCE(scx_switching_all);
 	int kind, cpu;
 
 	kind = atomic_read(&sch->exit_kind);
@@ -3976,6 +4039,22 @@ static void scx_disable_workfn(struct kthread_work *work)
 
 	scx_init_task_enabled = false;
 
+	for_each_online_cpu(cpu) {
+		struct rq *rq = cpu_rq(cpu);
+
+		/*
+		 * Invalidate all the rq clocks to prevent getting outdated
+		 * rq clocks from a previous scx scheduler.
+		 */
+		scx_rq_clock_invalidate(rq);
+
+		/*
+		 * We are unloading the sched_ext scheduler, we do not need its
+		 * DL server bandwidth anymore, remove it for all CPUs.
+		 */
+		dl_server_off(rq, is_switching_all);
+	}
+
 	scx_task_iter_start(&sti);
 	while ((p = scx_task_iter_next_locked(&sti))) {
 		unsigned int queue_flags = DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;
@@ -3997,15 +4076,6 @@ static void scx_disable_workfn(struct kthread_work *work)
 	scx_task_iter_stop(&sti);
 	percpu_up_write(&scx_fork_rwsem);
 
-	/*
-	 * Invalidate all the rq clocks to prevent getting outdated
-	 * rq clocks from a previous scx scheduler.
-	 */
-	for_each_possible_cpu(cpu) {
-		struct rq *rq = cpu_rq(cpu);
-		scx_rq_clock_invalidate(rq);
-	}
-
 	/* no task is on scx, turn off all the switches and flush in-progress calls */
 	static_branch_disable(&__scx_enabled);
 	bitmap_zero(sch->has_op, SCX_OPI_END);
@@ -4778,6 +4848,13 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 		put_task_struct(p);
 	}
 	scx_task_iter_stop(&sti);
+
+	/*
+	 * Enable the ext DL server on all online CPUs.
+	 */
+	for_each_online_cpu(cpu)
+		dl_server_on(cpu_rq(cpu), !(ops->flags & SCX_OPS_SWITCH_PARTIAL));
+
 	percpu_up_write(&scx_fork_rwsem);
 
 	scx_bypass(false);
-- 
2.51.0

[PATCH 13/14] selftests/sched_ext: Add test for sched_ext dl_server

[Andrea Righi]

Add a selftest to validate the correct behavior of the deadline server
for the ext_sched_class.

[ Joel: Replaced occurences of CFS in the test with EXT. ]

Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 tools/testing/selftests/sched_ext/Makefile    |   1 +
 .../selftests/sched_ext/rt_stall.bpf.c        |  23 ++
 tools/testing/selftests/sched_ext/rt_stall.c  | 214 ++++++++++++++++++
 3 files changed, 238 insertions(+)
 create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c
 create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c

diff --git a/tools/testing/selftests/sched_ext/Makefile b/tools/testing/selftests/sched_ext/Makefile
index 5fe45f9c5f8fd..c9255d1499b6e 100644
--- a/tools/testing/selftests/sched_ext/Makefile
+++ b/tools/testing/selftests/sched_ext/Makefile
@@ -183,6 +183,7 @@ auto-test-targets :=			\
 	select_cpu_dispatch_bad_dsq	\
 	select_cpu_dispatch_dbl_dsp	\
 	select_cpu_vtime		\
+	rt_stall			\
 	test_example			\
 
 testcase-targets := $(addsuffix .o,$(addprefix $(SCXOBJ_DIR)/,$(auto-test-targets)))
diff --git a/tools/testing/selftests/sched_ext/rt_stall.bpf.c b/tools/testing/selftests/sched_ext/rt_stall.bpf.c
new file mode 100644
index 0000000000000..80086779dd1eb
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/rt_stall.bpf.c
@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A scheduler that verified if RT tasks can stall SCHED_EXT tasks.
+ *
+ * Copyright (c) 2025 NVIDIA Corporation.
+ */
+
+#include <scx/common.bpf.h>
+
+char _license[] SEC("license") = "GPL";
+
+UEI_DEFINE(uei);
+
+void BPF_STRUCT_OPS(rt_stall_exit, struct scx_exit_info *ei)
+{
+	UEI_RECORD(uei, ei);
+}
+
+SEC(".struct_ops.link")
+struct sched_ext_ops rt_stall_ops = {
+	.exit			= (void *)rt_stall_exit,
+	.name			= "rt_stall",
+};
diff --git a/tools/testing/selftests/sched_ext/rt_stall.c b/tools/testing/selftests/sched_ext/rt_stall.c
new file mode 100644
index 0000000000000..e9a0def9ee323
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/rt_stall.c
@@ -0,0 +1,214 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2025 NVIDIA Corporation.
+ */
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sched.h>
+#include <sys/prctl.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <time.h>
+#include <linux/sched.h>
+#include <signal.h>
+#include <bpf/bpf.h>
+#include <scx/common.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "rt_stall.bpf.skel.h"
+#include "scx_test.h"
+#include "../kselftest.h"
+
+#define CORE_ID		0	/* CPU to pin tasks to */
+#define RUN_TIME        5	/* How long to run the test in seconds */
+
+/* Simple busy-wait function for test tasks */
+static void process_func(void)
+{
+	while (1) {
+		/* Busy wait */
+		for (volatile unsigned long i = 0; i < 10000000UL; i++)
+			;
+	}
+}
+
+/* Set CPU affinity to a specific core */
+static void set_affinity(int cpu)
+{
+	cpu_set_t mask;
+
+	CPU_ZERO(&mask);
+	CPU_SET(cpu, &mask);
+	if (sched_setaffinity(0, sizeof(mask), &mask) != 0) {
+		perror("sched_setaffinity");
+		exit(EXIT_FAILURE);
+	}
+}
+
+/* Set task scheduling policy and priority */
+static void set_sched(int policy, int priority)
+{
+	struct sched_param param;
+
+	param.sched_priority = priority;
+	if (sched_setscheduler(0, policy, &param) != 0) {
+		perror("sched_setscheduler");
+		exit(EXIT_FAILURE);
+	}
+}
+
+/* Get process runtime from /proc/<pid>/stat */
+static float get_process_runtime(int pid)
+{
+	char path[256];
+	FILE *file;
+	long utime, stime;
+	int fields;
+
+	snprintf(path, sizeof(path), "/proc/%d/stat", pid);
+	file = fopen(path, "r");
+	if (file == NULL) {
+		perror("Failed to open stat file");
+		return -1;
+	}
+
+	/* Skip the first 13 fields and read the 14th and 15th */
+	fields = fscanf(file,
+			"%*d %*s %*c %*d %*d %*d %*d %*d %*u %*u %*u %*u %*u %lu %lu",
+			&utime, &stime);
+	fclose(file);
+
+	if (fields != 2) {
+		fprintf(stderr, "Failed to read stat file\n");
+		return -1;
+	}
+
+	/* Calculate the total time spent in the process */
+	long total_time = utime + stime;
+	long ticks_per_second = sysconf(_SC_CLK_TCK);
+	float runtime_seconds = total_time * 1.0 / ticks_per_second;
+
+	return runtime_seconds;
+}
+
+static enum scx_test_status setup(void **ctx)
+{
+	struct rt_stall *skel;
+
+	skel = rt_stall__open();
+	SCX_FAIL_IF(!skel, "Failed to open");
+	SCX_ENUM_INIT(skel);
+	SCX_FAIL_IF(rt_stall__load(skel), "Failed to load skel");
+
+	*ctx = skel;
+
+	return SCX_TEST_PASS;
+}
+
+static bool sched_stress_test(void)
+{
+	float cfs_runtime, rt_runtime, actual_ratio;
+	int cfs_pid, rt_pid;
+	float expected_min_ratio = 0.04; /* 4% */
+
+	ksft_print_header();
+	ksft_set_plan(1);
+
+	/* Create and set up a EXT task */
+	cfs_pid = fork();
+	if (cfs_pid == 0) {
+		set_affinity(CORE_ID);
+		process_func();
+		exit(0);
+	} else if (cfs_pid < 0) {
+		perror("fork for EXT task");
+		ksft_exit_fail();
+	}
+
+	/* Create an RT task */
+	rt_pid = fork();
+	if (rt_pid == 0) {
+		set_affinity(CORE_ID);
+		set_sched(SCHED_FIFO, 50);
+		process_func();
+		exit(0);
+	} else if (rt_pid < 0) {
+		perror("fork for RT task");
+		ksft_exit_fail();
+	}
+
+	/* Let the processes run for the specified time */
+	sleep(RUN_TIME);
+
+	/* Get runtime for the EXT task */
+	cfs_runtime = get_process_runtime(cfs_pid);
+	if (cfs_runtime != -1)
+		ksft_print_msg("Runtime of EXT task (PID %d) is %f seconds\n",
+			       cfs_pid, cfs_runtime);
+	else
+		ksft_exit_fail_msg("Error getting runtime for EXT task (PID %d)\n", cfs_pid);
+
+	/* Get runtime for the RT task */
+	rt_runtime = get_process_runtime(rt_pid);
+	if (rt_runtime != -1)
+		ksft_print_msg("Runtime of RT task (PID %d) is %f seconds\n", rt_pid, rt_runtime);
+	else
+		ksft_exit_fail_msg("Error getting runtime for RT task (PID %d)\n", rt_pid);
+
+	/* Kill the processes */
+	kill(cfs_pid, SIGKILL);
+	kill(rt_pid, SIGKILL);
+	waitpid(cfs_pid, NULL, 0);
+	waitpid(rt_pid, NULL, 0);
+
+	/* Verify that the scx task got enough runtime */
+	actual_ratio = cfs_runtime / (cfs_runtime + rt_runtime);
+	ksft_print_msg("EXT task got %.2f%% of total runtime\n", actual_ratio * 100);
+
+	if (actual_ratio >= expected_min_ratio) {
+		ksft_test_result_pass("PASS: EXT task got more than %.2f%% of runtime\n",
+				      expected_min_ratio * 100);
+		return true;
+	}
+	ksft_test_result_fail("FAIL: EXT task got less than %.2f%% of runtime\n",
+			      expected_min_ratio * 100);
+	return false;
+}
+
+static enum scx_test_status run(void *ctx)
+{
+	struct rt_stall *skel = ctx;
+	struct bpf_link *link;
+	bool res;
+
+	link = bpf_map__attach_struct_ops(skel->maps.rt_stall_ops);
+	SCX_FAIL_IF(!link, "Failed to attach scheduler");
+
+	res = sched_stress_test();
+
+	SCX_EQ(skel->data->uei.kind, EXIT_KIND(SCX_EXIT_NONE));
+	bpf_link__destroy(link);
+
+	if (!res)
+		ksft_exit_fail();
+
+	return SCX_TEST_PASS;
+}
+
+static void cleanup(void *ctx)
+{
+	struct rt_stall *skel = ctx;
+
+	rt_stall__destroy(skel);
+}
+
+struct scx_test rt_stall = {
+	.name = "rt_stall",
+	.description = "Verify that RT tasks cannot stall SCHED_EXT tasks",
+	.setup = setup,
+	.run = run,
+	.cleanup = cleanup,
+};
+REGISTER_SCX_TEST(&rt_stall)
-- 
2.51.0

Re: [PATCH 13/14] selftests/sched_ext: Add test for sched_ext dl_server

[Emil Tsalapatis]

On Fri, Oct 17, 2025 at 5:38 AM Andrea Righi <arighi@nvidia.com> wrote:
>
> Add a selftest to validate the correct behavior of the deadline server
> for the ext_sched_class.
>
> [ Joel: Replaced occurences of CFS in the test with EXT. ]
>
> Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---

Nits listed below, but otherwise:
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

Code review aside, on my VM the test alternates between 4.81% and 5.20% for me
so it's working as expected.

>  tools/testing/selftests/sched_ext/Makefile    |   1 +
>  .../selftests/sched_ext/rt_stall.bpf.c        |  23 ++
>  tools/testing/selftests/sched_ext/rt_stall.c  | 214 ++++++++++++++++++
>  3 files changed, 238 insertions(+)
>  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c
>  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c
>
> diff --git a/tools/testing/selftests/sched_ext/Makefile b/tools/testing/selftests/sched_ext/Makefile
> index 5fe45f9c5f8fd..c9255d1499b6e 100644
> --- a/tools/testing/selftests/sched_ext/Makefile
> +++ b/tools/testing/selftests/sched_ext/Makefile
> @@ -183,6 +183,7 @@ auto-test-targets :=                        \
>         select_cpu_dispatch_bad_dsq     \
>         select_cpu_dispatch_dbl_dsp     \
>         select_cpu_vtime                \
> +       rt_stall                        \
>         test_example                    \
>
>  testcase-targets := $(addsuffix .o,$(addprefix $(SCXOBJ_DIR)/,$(auto-test-targets)))
> diff --git a/tools/testing/selftests/sched_ext/rt_stall.bpf.c b/tools/testing/selftests/sched_ext/rt_stall.bpf.c
> new file mode 100644
> index 0000000000000..80086779dd1eb
> --- /dev/null
> +++ b/tools/testing/selftests/sched_ext/rt_stall.bpf.c
> @@ -0,0 +1,23 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * A scheduler that verified if RT tasks can stall SCHED_EXT tasks.
> + *
> + * Copyright (c) 2025 NVIDIA Corporation.
> + */
> +
> +#include <scx/common.bpf.h>
> +
> +char _license[] SEC("license") = "GPL";
> +
> +UEI_DEFINE(uei);
> +
> +void BPF_STRUCT_OPS(rt_stall_exit, struct scx_exit_info *ei)
> +{
> +       UEI_RECORD(uei, ei);
> +}
> +
> +SEC(".struct_ops.link")
> +struct sched_ext_ops rt_stall_ops = {
> +       .exit                   = (void *)rt_stall_exit,
> +       .name                   = "rt_stall",
> +};
> diff --git a/tools/testing/selftests/sched_ext/rt_stall.c b/tools/testing/selftests/sched_ext/rt_stall.c
> new file mode 100644
> index 0000000000000..e9a0def9ee323
> --- /dev/null
> +++ b/tools/testing/selftests/sched_ext/rt_stall.c
> @@ -0,0 +1,214 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2025 NVIDIA Corporation.
> + */
> +#define _GNU_SOURCE
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <unistd.h>
> +#include <sched.h>
> +#include <sys/prctl.h>
> +#include <sys/types.h>
> +#include <sys/wait.h>
> +#include <time.h>
> +#include <linux/sched.h>
> +#include <signal.h>
> +#include <bpf/bpf.h>
> +#include <scx/common.h>
> +#include <sys/wait.h>
> +#include <unistd.h>
> +#include "rt_stall.bpf.skel.h"
> +#include "scx_test.h"
> +#include "../kselftest.h"
> +
> +#define CORE_ID                0       /* CPU to pin tasks to */
> +#define RUN_TIME        5      /* How long to run the test in seconds */
> +
> +/* Simple busy-wait function for test tasks */
> +static void process_func(void)
> +{
> +       while (1) {
> +               /* Busy wait */
> +               for (volatile unsigned long i = 0; i < 10000000UL; i++)
> +                       ;
> +       }
> +}
> +
> +/* Set CPU affinity to a specific core */
> +static void set_affinity(int cpu)
> +{
> +       cpu_set_t mask;
> +
> +       CPU_ZERO(&mask);
> +       CPU_SET(cpu, &mask);
> +       if (sched_setaffinity(0, sizeof(mask), &mask) != 0) {
> +               perror("sched_setaffinity");
> +               exit(EXIT_FAILURE);
> +       }
> +}
> +
> +/* Set task scheduling policy and priority */
> +static void set_sched(int policy, int priority)
> +{
> +       struct sched_param param;
> +
> +       param.sched_priority = priority;
> +       if (sched_setscheduler(0, policy, &param) != 0) {
> +               perror("sched_setscheduler");
> +               exit(EXIT_FAILURE);
> +       }
> +}
> +
> +/* Get process runtime from /proc/<pid>/stat */
> +static float get_process_runtime(int pid)
> +{
> +       char path[256];
> +       FILE *file;
> +       long utime, stime;
> +       int fields;
> +
> +       snprintf(path, sizeof(path), "/proc/%d/stat", pid);
> +       file = fopen(path, "r");
> +       if (file == NULL) {
> +               perror("Failed to open stat file");
> +               return -1;
> +       }
> +
> +       /* Skip the first 13 fields and read the 14th and 15th */
> +       fields = fscanf(file,
> +                       "%*d %*s %*c %*d %*d %*d %*d %*d %*u %*u %*u %*u %*u %lu %lu",
> +                       &utime, &stime);
> +       fclose(file);
> +
> +       if (fields != 2) {
> +               fprintf(stderr, "Failed to read stat file\n");
> +               return -1;
> +       }
> +
> +       /* Calculate the total time spent in the process */
> +       long total_time = utime + stime;
> +       long ticks_per_second = sysconf(_SC_CLK_TCK);
> +       float runtime_seconds = total_time * 1.0 / ticks_per_second;
> +
> +       return runtime_seconds;
> +}
> +
> +static enum scx_test_status setup(void **ctx)
> +{
> +       struct rt_stall *skel;
> +
> +       skel = rt_stall__open();
> +       SCX_FAIL_IF(!skel, "Failed to open");
> +       SCX_ENUM_INIT(skel);
> +       SCX_FAIL_IF(rt_stall__load(skel), "Failed to load skel");
> +
> +       *ctx = skel;
> +
> +       return SCX_TEST_PASS;
> +}
> +
> +static bool sched_stress_test(void)
> +{
> +       float cfs_runtime, rt_runtime, actual_ratio;
> +       int cfs_pid, rt_pid;

I think it should be cfs_pid -> ext_pid, cfs_runtime -> ext_runtime

> +       float expected_min_ratio = 0.04; /* 4% */

Maybe add a comment that explains the 4% value? As in, we're expecting
it to be around 5% so 0.04 accounts for values close enough but
below < 5%.

> +
> +       ksft_print_header();
> +       ksft_set_plan(1);
> +
> +       /* Create and set up a EXT task */
> +       cfs_pid = fork();
> +       if (cfs_pid == 0) {
> +               set_affinity(CORE_ID);
> +               process_func();
> +               exit(0);
> +       } else if (cfs_pid < 0) {
> +               perror("fork for EXT task");
> +               ksft_exit_fail();
> +       }
> +
> +       /* Create an RT task */
> +       rt_pid = fork();
> +       if (rt_pid == 0) {
> +               set_affinity(CORE_ID);
> +               set_sched(SCHED_FIFO, 50);
> +               process_func();
> +               exit(0);
> +       } else if (rt_pid < 0) {
> +               perror("fork for RT task");
> +               ksft_exit_fail();
> +       }
> +
> +       /* Let the processes run for the specified time */
> +       sleep(RUN_TIME);
> +
> +       /* Get runtime for the EXT task */
> +       cfs_runtime = get_process_runtime(cfs_pid);
> +       if (cfs_runtime != -1)
> +               ksft_print_msg("Runtime of EXT task (PID %d) is %f seconds\n",
> +                              cfs_pid, cfs_runtime);
> +       else
> +               ksft_exit_fail_msg("Error getting runtime for EXT task (PID %d)\n", cfs_pid);
> +
> +       /* Get runtime for the RT task */
> +       rt_runtime = get_process_runtime(rt_pid);
> +       if (rt_runtime != -1)
> +               ksft_print_msg("Runtime of RT task (PID %d) is %f seconds\n", rt_pid, rt_runtime);
> +       else
> +               ksft_exit_fail_msg("Error getting runtime for RT task (PID %d)\n", rt_pid);
> +

Minor, but why not

if (rt_runtime == -1)
        ksft_exit_fail_msg("Error getting runtime for RT task (PID
%d)\n", rt_pid);
ksft_print_msg("Runtime of RT task (PID %d) is %f seconds\n", rt_pid,
rt_runtime);

since ksft_exit_fail_msg never returns?

> +       /* Kill the processes */
> +       kill(cfs_pid, SIGKILL);
> +       kill(rt_pid, SIGKILL);
> +       waitpid(cfs_pid, NULL, 0);
> +       waitpid(rt_pid, NULL, 0);
> +
> +       /* Verify that the scx task got enough runtime */
> +       actual_ratio = cfs_runtime / (cfs_runtime + rt_runtime);
> +       ksft_print_msg("EXT task got %.2f%% of total runtime\n", actual_ratio * 100);
> +
> +       if (actual_ratio >= expected_min_ratio) {
> +               ksft_test_result_pass("PASS: EXT task got more than %.2f%% of runtime\n",
> +                                     expected_min_ratio * 100);
> +               return true;
> +       }
> +       ksft_test_result_fail("FAIL: EXT task got less than %.2f%% of runtime\n",
> +                             expected_min_ratio * 100);
> +       return false;
> +}
> +
> +static enum scx_test_status run(void *ctx)
> +{
> +       struct rt_stall *skel = ctx;
> +       struct bpf_link *link;
> +       bool res;
> +
> +       link = bpf_map__attach_struct_ops(skel->maps.rt_stall_ops);
> +       SCX_FAIL_IF(!link, "Failed to attach scheduler");
> +
> +       res = sched_stress_test();
> +
> +       SCX_EQ(skel->data->uei.kind, EXIT_KIND(SCX_EXIT_NONE));
> +       bpf_link__destroy(link);
> +
> +       if (!res)
> +               ksft_exit_fail();
> +
> +       return SCX_TEST_PASS;
> +}
> +
> +static void cleanup(void *ctx)
> +{
> +       struct rt_stall *skel = ctx;
> +
> +       rt_stall__destroy(skel);
> +}
> +
> +struct scx_test rt_stall = {
> +       .name = "rt_stall",
> +       .description = "Verify that RT tasks cannot stall SCHED_EXT tasks",
> +       .setup = setup,
> +       .run = run,
> +       .cleanup = cleanup,
> +};
> +REGISTER_SCX_TEST(&rt_stall)
> --
> 2.51.0
>
>

Re: [PATCH 13/14] selftests/sched_ext: Add test for sched_ext dl_server

[Andrea Righi]

Hi Emil,

On Sun, Oct 19, 2025 at 03:04:22PM -0400, Emil Tsalapatis wrote:
> On Fri, Oct 17, 2025 at 5:38 AM Andrea Righi <arighi@nvidia.com> wrote:
> >
> > Add a selftest to validate the correct behavior of the deadline server
> > for the ext_sched_class.
> >
> > [ Joel: Replaced occurences of CFS in the test with EXT. ]
> >
> > Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
> > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > ---
> 
> Nits listed below, but otherwise:
> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
> 
> Code review aside, on my VM the test alternates between 4.81% and 5.20% for me
> so it's working as expected.

Yeah, that sounds right, a bit of fluctuation like that is expected.

> 
> >  tools/testing/selftests/sched_ext/Makefile    |   1 +
> >  .../selftests/sched_ext/rt_stall.bpf.c        |  23 ++
> >  tools/testing/selftests/sched_ext/rt_stall.c  | 214 ++++++++++++++++++
> >  3 files changed, 238 insertions(+)
> >  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c
> >  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c
> >
> > diff --git a/tools/testing/selftests/sched_ext/Makefile b/tools/testing/selftests/sched_ext/Makefile
> > index 5fe45f9c5f8fd..c9255d1499b6e 100644
> > --- a/tools/testing/selftests/sched_ext/Makefile
> > +++ b/tools/testing/selftests/sched_ext/Makefile
> > @@ -183,6 +183,7 @@ auto-test-targets :=                        \
> >         select_cpu_dispatch_bad_dsq     \
> >         select_cpu_dispatch_dbl_dsp     \
> >         select_cpu_vtime                \
> > +       rt_stall                        \
> >         test_example                    \
> >
> >  testcase-targets := $(addsuffix .o,$(addprefix $(SCXOBJ_DIR)/,$(auto-test-targets)))
> > diff --git a/tools/testing/selftests/sched_ext/rt_stall.bpf.c b/tools/testing/selftests/sched_ext/rt_stall.bpf.c
> > new file mode 100644
> > index 0000000000000..80086779dd1eb
> > --- /dev/null
> > +++ b/tools/testing/selftests/sched_ext/rt_stall.bpf.c
> > @@ -0,0 +1,23 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * A scheduler that verified if RT tasks can stall SCHED_EXT tasks.
> > + *
> > + * Copyright (c) 2025 NVIDIA Corporation.
> > + */
> > +
> > +#include <scx/common.bpf.h>
> > +
> > +char _license[] SEC("license") = "GPL";
> > +
> > +UEI_DEFINE(uei);
> > +
> > +void BPF_STRUCT_OPS(rt_stall_exit, struct scx_exit_info *ei)
> > +{
> > +       UEI_RECORD(uei, ei);
> > +}
> > +
> > +SEC(".struct_ops.link")
> > +struct sched_ext_ops rt_stall_ops = {
> > +       .exit                   = (void *)rt_stall_exit,
> > +       .name                   = "rt_stall",
> > +};
> > diff --git a/tools/testing/selftests/sched_ext/rt_stall.c b/tools/testing/selftests/sched_ext/rt_stall.c
> > new file mode 100644
> > index 0000000000000..e9a0def9ee323
> > --- /dev/null
> > +++ b/tools/testing/selftests/sched_ext/rt_stall.c
> > @@ -0,0 +1,214 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * Copyright (c) 2025 NVIDIA Corporation.
> > + */
> > +#define _GNU_SOURCE
> > +#include <stdio.h>
> > +#include <stdlib.h>
> > +#include <unistd.h>
> > +#include <sched.h>
> > +#include <sys/prctl.h>
> > +#include <sys/types.h>
> > +#include <sys/wait.h>
> > +#include <time.h>
> > +#include <linux/sched.h>
> > +#include <signal.h>
> > +#include <bpf/bpf.h>
> > +#include <scx/common.h>
> > +#include <sys/wait.h>
> > +#include <unistd.h>
> > +#include "rt_stall.bpf.skel.h"
> > +#include "scx_test.h"
> > +#include "../kselftest.h"
> > +
> > +#define CORE_ID                0       /* CPU to pin tasks to */
> > +#define RUN_TIME        5      /* How long to run the test in seconds */
> > +
> > +/* Simple busy-wait function for test tasks */
> > +static void process_func(void)
> > +{
> > +       while (1) {
> > +               /* Busy wait */
> > +               for (volatile unsigned long i = 0; i < 10000000UL; i++)
> > +                       ;
> > +       }
> > +}
> > +
> > +/* Set CPU affinity to a specific core */
> > +static void set_affinity(int cpu)
> > +{
> > +       cpu_set_t mask;
> > +
> > +       CPU_ZERO(&mask);
> > +       CPU_SET(cpu, &mask);
> > +       if (sched_setaffinity(0, sizeof(mask), &mask) != 0) {
> > +               perror("sched_setaffinity");
> > +               exit(EXIT_FAILURE);
> > +       }
> > +}
> > +
> > +/* Set task scheduling policy and priority */
> > +static void set_sched(int policy, int priority)
> > +{
> > +       struct sched_param param;
> > +
> > +       param.sched_priority = priority;
> > +       if (sched_setscheduler(0, policy, &param) != 0) {
> > +               perror("sched_setscheduler");
> > +               exit(EXIT_FAILURE);
> > +       }
> > +}
> > +
> > +/* Get process runtime from /proc/<pid>/stat */
> > +static float get_process_runtime(int pid)
> > +{
> > +       char path[256];
> > +       FILE *file;
> > +       long utime, stime;
> > +       int fields;
> > +
> > +       snprintf(path, sizeof(path), "/proc/%d/stat", pid);
> > +       file = fopen(path, "r");
> > +       if (file == NULL) {
> > +               perror("Failed to open stat file");
> > +               return -1;
> > +       }
> > +
> > +       /* Skip the first 13 fields and read the 14th and 15th */
> > +       fields = fscanf(file,
> > +                       "%*d %*s %*c %*d %*d %*d %*d %*d %*u %*u %*u %*u %*u %lu %lu",
> > +                       &utime, &stime);
> > +       fclose(file);
> > +
> > +       if (fields != 2) {
> > +               fprintf(stderr, "Failed to read stat file\n");
> > +               return -1;
> > +       }
> > +
> > +       /* Calculate the total time spent in the process */
> > +       long total_time = utime + stime;
> > +       long ticks_per_second = sysconf(_SC_CLK_TCK);
> > +       float runtime_seconds = total_time * 1.0 / ticks_per_second;
> > +
> > +       return runtime_seconds;
> > +}
> > +
> > +static enum scx_test_status setup(void **ctx)
> > +{
> > +       struct rt_stall *skel;
> > +
> > +       skel = rt_stall__open();
> > +       SCX_FAIL_IF(!skel, "Failed to open");
> > +       SCX_ENUM_INIT(skel);
> > +       SCX_FAIL_IF(rt_stall__load(skel), "Failed to load skel");
> > +
> > +       *ctx = skel;
> > +
> > +       return SCX_TEST_PASS;
> > +}
> > +
> > +static bool sched_stress_test(void)
> > +{
> > +       float cfs_runtime, rt_runtime, actual_ratio;
> > +       int cfs_pid, rt_pid;
> 
> I think it should be cfs_pid -> ext_pid, cfs_runtime -> ext_runtime
> 
> > +       float expected_min_ratio = 0.04; /* 4% */
> 
> Maybe add a comment that explains the 4% value? As in, we're expecting
> it to be around 5% so 0.04 accounts for values close enough but
> below < 5%.

Makes sense, I’ll add this comment (or something along those lines).

> 
> > +
> > +       ksft_print_header();
> > +       ksft_set_plan(1);
> > +
> > +       /* Create and set up a EXT task */
> > +       cfs_pid = fork();
> > +       if (cfs_pid == 0) {
> > +               set_affinity(CORE_ID);
> > +               process_func();
> > +               exit(0);
> > +       } else if (cfs_pid < 0) {
> > +               perror("fork for EXT task");
> > +               ksft_exit_fail();
> > +       }
> > +
> > +       /* Create an RT task */
> > +       rt_pid = fork();
> > +       if (rt_pid == 0) {
> > +               set_affinity(CORE_ID);
> > +               set_sched(SCHED_FIFO, 50);
> > +               process_func();
> > +               exit(0);
> > +       } else if (rt_pid < 0) {
> > +               perror("fork for RT task");
> > +               ksft_exit_fail();
> > +       }
> > +
> > +       /* Let the processes run for the specified time */
> > +       sleep(RUN_TIME);
> > +
> > +       /* Get runtime for the EXT task */
> > +       cfs_runtime = get_process_runtime(cfs_pid);
> > +       if (cfs_runtime != -1)
> > +               ksft_print_msg("Runtime of EXT task (PID %d) is %f seconds\n",
> > +                              cfs_pid, cfs_runtime);
> > +       else
> > +               ksft_exit_fail_msg("Error getting runtime for EXT task (PID %d)\n", cfs_pid);
> > +
> > +       /* Get runtime for the RT task */
> > +       rt_runtime = get_process_runtime(rt_pid);
> > +       if (rt_runtime != -1)
> > +               ksft_print_msg("Runtime of RT task (PID %d) is %f seconds\n", rt_pid, rt_runtime);
> > +       else
> > +               ksft_exit_fail_msg("Error getting runtime for RT task (PID %d)\n", rt_pid);
> > +
> 
> Minor, but why not
> 
> if (rt_runtime == -1)
>         ksft_exit_fail_msg("Error getting runtime for RT task (PID
> %d)\n", rt_pid);
> ksft_print_msg("Runtime of RT task (PID %d) is %f seconds\n", rt_pid,
> rt_runtime);
> 
> since ksft_exit_fail_msg never returns?

Ack.

> 
> > +       /* Kill the processes */
> > +       kill(cfs_pid, SIGKILL);
> > +       kill(rt_pid, SIGKILL);
> > +       waitpid(cfs_pid, NULL, 0);
> > +       waitpid(rt_pid, NULL, 0);
> > +
> > +       /* Verify that the scx task got enough runtime */
> > +       actual_ratio = cfs_runtime / (cfs_runtime + rt_runtime);
> > +       ksft_print_msg("EXT task got %.2f%% of total runtime\n", actual_ratio * 100);
> > +
> > +       if (actual_ratio >= expected_min_ratio) {
> > +               ksft_test_result_pass("PASS: EXT task got more than %.2f%% of runtime\n",
> > +                                     expected_min_ratio * 100);
> > +               return true;
> > +       }
> > +       ksft_test_result_fail("FAIL: EXT task got less than %.2f%% of runtime\n",
> > +                             expected_min_ratio * 100);
> > +       return false;
> > +}
> > +
> > +static enum scx_test_status run(void *ctx)
> > +{
> > +       struct rt_stall *skel = ctx;
> > +       struct bpf_link *link;
> > +       bool res;
> > +
> > +       link = bpf_map__attach_struct_ops(skel->maps.rt_stall_ops);
> > +       SCX_FAIL_IF(!link, "Failed to attach scheduler");
> > +
> > +       res = sched_stress_test();
> > +
> > +       SCX_EQ(skel->data->uei.kind, EXIT_KIND(SCX_EXIT_NONE));
> > +       bpf_link__destroy(link);
> > +
> > +       if (!res)
> > +               ksft_exit_fail();
> > +
> > +       return SCX_TEST_PASS;
> > +}
> > +
> > +static void cleanup(void *ctx)
> > +{
> > +       struct rt_stall *skel = ctx;
> > +
> > +       rt_stall__destroy(skel);
> > +}
> > +
> > +struct scx_test rt_stall = {
> > +       .name = "rt_stall",
> > +       .description = "Verify that RT tasks cannot stall SCHED_EXT tasks",
> > +       .setup = setup,
> > +       .run = run,
> > +       .cleanup = cleanup,
> > +};
> > +REGISTER_SCX_TEST(&rt_stall)
> > --
> > 2.51.0
> >
> >

Thanks,
-Andrea

Re: [PATCH 13/14] selftests/sched_ext: Add test for sched_ext dl_server

[Andrea Righi]

On Mon, Oct 20, 2025 at 03:22:53PM +0200, Andrea Righi wrote:
> Hi Emil,
...
> > > +static bool sched_stress_test(void)
> > > +{
> > > +       float cfs_runtime, rt_runtime, actual_ratio;
> > > +       int cfs_pid, rt_pid;
> > 
> > I think it should be cfs_pid -> ext_pid, cfs_runtime -> ext_runtime

Oh and ack to this as well, using CFS in general is a bit confusing at this
point.

-Andrea

Re: [PATCH 13/14] selftests/sched_ext: Add test for sched_ext dl_server

[Christian Loehle]

On 10/17/25 10:26, Andrea Righi wrote:
> Add a selftest to validate the correct behavior of the deadline server
> for the ext_sched_class.
> 
> [ Joel: Replaced occurences of CFS in the test with EXT. ]
> 
> Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  tools/testing/selftests/sched_ext/Makefile    |   1 +
>  .../selftests/sched_ext/rt_stall.bpf.c        |  23 ++
>  tools/testing/selftests/sched_ext/rt_stall.c  | 214 ++++++++++++++++++
>  3 files changed, 238 insertions(+)
>  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c
>  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c


Does this pass consistently for you?
For a loop of 1000 runs I'm getting total runtime numbers for the EXT task of:

   0.000 -    0.261 |  (7)
   0.261 -    0.522 | ###### (86)
   0.522 -    4.437 |  (0)
   4.437 -    4.698 |  (1)
   4.698 -    4.959 | ################### (257)
   4.959 -    5.220 | ################################################## (649)

I'll try to see what's going wrong here...

Re: [PATCH 13/14] selftests/sched_ext: Add test for sched_ext dl_server

[Andrea Righi]

Hi Christian,

On Mon, Oct 20, 2025 at 02:26:17PM +0100, Christian Loehle wrote:
> On 10/17/25 10:26, Andrea Righi wrote:
> > Add a selftest to validate the correct behavior of the deadline server
> > for the ext_sched_class.
> > 
> > [ Joel: Replaced occurences of CFS in the test with EXT. ]
> > 
> > Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
> > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > ---
> >  tools/testing/selftests/sched_ext/Makefile    |   1 +
> >  .../selftests/sched_ext/rt_stall.bpf.c        |  23 ++
> >  tools/testing/selftests/sched_ext/rt_stall.c  | 214 ++++++++++++++++++
> >  3 files changed, 238 insertions(+)
> >  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c
> >  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c
> 
> 
> Does this pass consistently for you?
> For a loop of 1000 runs I'm getting total runtime numbers for the EXT task of:
> 
>    0.000 -    0.261 |  (7)
>    0.261 -    0.522 | ###### (86)
>    0.522 -    4.437 |  (0)
>    4.437 -    4.698 |  (1)
>    4.698 -    4.959 | ################### (257)
>    4.959 -    5.220 | ################################################## (649)
> 
> I'll try to see what's going wrong here...

Is that 1000 runs of total_bw? Yeah, the small ones don't look right at
all, unless they're caused by some errors in the measurement (or something
wrong in the test itself). Still better than without the dl_server, but
it'd be nice to understand what's going on. :)

I'll try to reproduce that on my side as well.

Thanks,
-Andrea

Re: [PATCH 13/14] selftests/sched_ext: Add test for sched_ext dl_server

[Andrea Righi]

On Mon, Oct 20, 2025 at 03:55:52PM +0200, Andrea Righi wrote:
> Hi Christian,
> 
> On Mon, Oct 20, 2025 at 02:26:17PM +0100, Christian Loehle wrote:
> > On 10/17/25 10:26, Andrea Righi wrote:
> > > Add a selftest to validate the correct behavior of the deadline server
> > > for the ext_sched_class.
> > > 
> > > [ Joel: Replaced occurences of CFS in the test with EXT. ]
> > > 
> > > Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
> > > Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > > Signed-off-by: Andrea Righi <arighi@nvidia.com>
> > > ---
> > >  tools/testing/selftests/sched_ext/Makefile    |   1 +
> > >  .../selftests/sched_ext/rt_stall.bpf.c        |  23 ++
> > >  tools/testing/selftests/sched_ext/rt_stall.c  | 214 ++++++++++++++++++
> > >  3 files changed, 238 insertions(+)
> > >  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c
> > >  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c
> > 
> > 
> > Does this pass consistently for you?
> > For a loop of 1000 runs I'm getting total runtime numbers for the EXT task of:
> > 
> >    0.000 -    0.261 |  (7)
> >    0.261 -    0.522 | ###### (86)
> >    0.522 -    4.437 |  (0)
> >    4.437 -    4.698 |  (1)
> >    4.698 -    4.959 | ################### (257)
> >    4.959 -    5.220 | ################################################## (649)
> > 
> > I'll try to see what's going wrong here...
> 
> Is that 1000 runs of total_bw? Yeah, the small ones don't look right at

s/total_bw/rt_stall/

-Andrea

> all, unless they're caused by some errors in the measurement (or something
> wrong in the test itself). Still better than without the dl_server, but
> it'd be nice to understand what's going on. :)
> 
> I'll try to reproduce that on my side as well.
> 
> Thanks,
> -Andrea

Re: [PATCH 13/14] selftests/sched_ext: Add test for sched_ext dl_server

[Christian Loehle]

On 10/20/25 14:55, Andrea Righi wrote:
> Hi Christian,
> 
> On Mon, Oct 20, 2025 at 02:26:17PM +0100, Christian Loehle wrote:
>> On 10/17/25 10:26, Andrea Righi wrote:
>>> Add a selftest to validate the correct behavior of the deadline server
>>> for the ext_sched_class.
>>>
>>> [ Joel: Replaced occurences of CFS in the test with EXT. ]
>>>
>>> Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
>>> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
>>> Signed-off-by: Andrea Righi <arighi@nvidia.com>
>>> ---
>>>  tools/testing/selftests/sched_ext/Makefile    |   1 +
>>>  .../selftests/sched_ext/rt_stall.bpf.c        |  23 ++
>>>  tools/testing/selftests/sched_ext/rt_stall.c  | 214 ++++++++++++++++++
>>>  3 files changed, 238 insertions(+)
>>>  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c
>>>  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c
>>
>>
>> Does this pass consistently for you?
>> For a loop of 1000 runs I'm getting total runtime numbers for the EXT task of:
>>
>>    0.000 -    0.261 |  (7)
>>    0.261 -    0.522 | ###### (86)
>>    0.522 -    4.437 |  (0)
>>    4.437 -    4.698 |  (1)
>>    4.698 -    4.959 | ################### (257)
>>    4.959 -    5.220 | ################################################## (649)
>>
>> I'll try to see what's going wrong here...
> 
> Is that 1000 runs of total_bw? Yeah, the small ones don't look right at
> all, unless they're caused by some errors in the measurement (or something
> wrong in the test itself). Still better than without the dl_server, but
> it'd be nice to understand what's going on. :)
> 
> I'll try to reproduce that on my side as well.
> 

Yes it's pretty much
for i in $(seq 0 999); do ./runner -t rt_stall ; sleep 10; done

I also tried to increase the runtime of the test, but results look the same so I
assume the DL server isn't running in the fail cases.

Re: [PATCH 13/14] selftests/sched_ext: Add test for sched_ext dl_server

[Christian Loehle]

On 10/20/25 15:21, Christian Loehle wrote:
> On 10/20/25 14:55, Andrea Righi wrote:
>> Hi Christian,
>>
>> On Mon, Oct 20, 2025 at 02:26:17PM +0100, Christian Loehle wrote:
>>> On 10/17/25 10:26, Andrea Righi wrote:
>>>> Add a selftest to validate the correct behavior of the deadline server
>>>> for the ext_sched_class.
>>>>
>>>> [ Joel: Replaced occurences of CFS in the test with EXT. ]
>>>>
>>>> Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
>>>> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
>>>> Signed-off-by: Andrea Righi <arighi@nvidia.com>
>>>> ---
>>>>  tools/testing/selftests/sched_ext/Makefile    |   1 +
>>>>  .../selftests/sched_ext/rt_stall.bpf.c        |  23 ++
>>>>  tools/testing/selftests/sched_ext/rt_stall.c  | 214 ++++++++++++++++++
>>>>  3 files changed, 238 insertions(+)
>>>>  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c
>>>>  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c
>>>
>>>
>>> Does this pass consistently for you?
>>> For a loop of 1000 runs I'm getting total runtime numbers for the EXT task of:
>>>
>>>    0.000 -    0.261 |  (7)
>>>    0.261 -    0.522 | ###### (86)
>>>    0.522 -    4.437 |  (0)
>>>    4.437 -    4.698 |  (1)
>>>    4.698 -    4.959 | ################### (257)
>>>    4.959 -    5.220 | ################################################## (649)
>>>
>>> I'll try to see what's going wrong here...
>>
>> Is that 1000 runs of total_bw? Yeah, the small ones don't look right at
>> all, unless they're caused by some errors in the measurement (or something
>> wrong in the test itself). Still better than without the dl_server, but
>> it'd be nice to understand what's going on. :)
>>
>> I'll try to reproduce that on my side as well.
>>
> 
> Yes it's pretty much
> for i in $(seq 0 999); do ./runner -t rt_stall ; sleep 10; done
> 
> I also tried to increase the runtime of the test, but results look the same so I
> assume the DL server isn't running in the fail cases.
> 

FWIW the below fixes the issue and also explains why runtime of the test was irrelevant.
I wonder if we should let the test do FAIR->EXT->FAIR->EXT or something like that,
the change would be minimal and coverage improved significantly IMO.

-----8<-----

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index c5f3c39972b6..ed48c681c4c2 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2568,6 +2568,8 @@ static void dl_server_on(struct rq *rq, bool switch_all)
 
        err = dl_server_init_params(&rq->ext_server);
        WARN_ON_ONCE(err);
+       if (rq->scx.nr_running)
+               dl_server_start(&rq->ext_server);
 
        rq_unlock_irqrestore(rq, &rf);
 }

Re: [PATCH 13/14] selftests/sched_ext: Add test for sched_ext dl_server

[Andrea Righi]

On Thu, Oct 23, 2025 at 04:01:59PM +0100, Christian Loehle wrote:
> On 10/20/25 15:21, Christian Loehle wrote:
> > On 10/20/25 14:55, Andrea Righi wrote:
> >> Hi Christian,
> >>
> >> On Mon, Oct 20, 2025 at 02:26:17PM +0100, Christian Loehle wrote:
> >>> On 10/17/25 10:26, Andrea Righi wrote:
> >>>> Add a selftest to validate the correct behavior of the deadline server
> >>>> for the ext_sched_class.
> >>>>
> >>>> [ Joel: Replaced occurences of CFS in the test with EXT. ]
> >>>>
> >>>> Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
> >>>> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> >>>> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> >>>> ---
> >>>>  tools/testing/selftests/sched_ext/Makefile    |   1 +
> >>>>  .../selftests/sched_ext/rt_stall.bpf.c        |  23 ++
> >>>>  tools/testing/selftests/sched_ext/rt_stall.c  | 214 ++++++++++++++++++
> >>>>  3 files changed, 238 insertions(+)
> >>>>  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c
> >>>>  create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c
> >>>
> >>>
> >>> Does this pass consistently for you?
> >>> For a loop of 1000 runs I'm getting total runtime numbers for the EXT task of:
> >>>
> >>>    0.000 -    0.261 |  (7)
> >>>    0.261 -    0.522 | ###### (86)
> >>>    0.522 -    4.437 |  (0)
> >>>    4.437 -    4.698 |  (1)
> >>>    4.698 -    4.959 | ################### (257)
> >>>    4.959 -    5.220 | ################################################## (649)
> >>>
> >>> I'll try to see what's going wrong here...
> >>
> >> Is that 1000 runs of total_bw? Yeah, the small ones don't look right at
> >> all, unless they're caused by some errors in the measurement (or something
> >> wrong in the test itself). Still better than without the dl_server, but
> >> it'd be nice to understand what's going on. :)
> >>
> >> I'll try to reproduce that on my side as well.
> >>
> > 
> > Yes it's pretty much
> > for i in $(seq 0 999); do ./runner -t rt_stall ; sleep 10; done
> > 
> > I also tried to increase the runtime of the test, but results look the same so I
> > assume the DL server isn't running in the fail cases.
> > 
> 
> FWIW the below fixes the issue and also explains why runtime of the test was irrelevant.

Ah, good catch Christian! this makes sense to me, I'll also run some tests
on my side with this applied and I'll include it in the next patch series.

> I wonder if we should let the test do FAIR->EXT->FAIR->EXT or something like that,
> the change would be minimal and coverage improved significantly IMO.

I agree, running a couple of rounds of fair->ext seems reasonable to me and
it can potentially trigger more issues in advance.

Thanks,
-Andrea

> 
> -----8<-----
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index c5f3c39972b6..ed48c681c4c2 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -2568,6 +2568,8 @@ static void dl_server_on(struct rq *rq, bool switch_all)
>  
>         err = dl_server_init_params(&rq->ext_server);
>         WARN_ON_ONCE(err);
> +       if (rq->scx.nr_running)
> +               dl_server_start(&rq->ext_server);
>  
>         rq_unlock_irqrestore(rq, &rf);
>  }
> 

[PATCH 14/14] selftests/sched_ext: Add test for DL server total_bw consistency

[Andrea Righi]

From: Joel Fernandes <joelagnelf@nvidia.com>

Add a new kselftest to verify that the total_bw value in
/sys/kernel/debug/sched/debug remains consistent across all CPUs
under different sched_ext BPF program states:

1. Before a BPF scheduler is loaded
2. While a BPF scheduler is loaded and active
3. After a BPF scheduler is unloaded

The test runs CPU stress threads to ensure DL server bandwidth
values stabilize before checking consistency. This helps catch
potential issues with DL server bandwidth accounting during
sched_ext transitions.

[ arighi: small coding style fixes ]

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 tools/testing/selftests/sched_ext/Makefile   |   1 +
 tools/testing/selftests/sched_ext/total_bw.c | 281 +++++++++++++++++++
 2 files changed, 282 insertions(+)
 create mode 100644 tools/testing/selftests/sched_ext/total_bw.c

diff --git a/tools/testing/selftests/sched_ext/Makefile b/tools/testing/selftests/sched_ext/Makefile
index c9255d1499b6e..2c601a7eaff5f 100644
--- a/tools/testing/selftests/sched_ext/Makefile
+++ b/tools/testing/selftests/sched_ext/Makefile
@@ -185,6 +185,7 @@ auto-test-targets :=			\
 	select_cpu_vtime		\
 	rt_stall			\
 	test_example			\
+	total_bw			\
 
 testcase-targets := $(addsuffix .o,$(addprefix $(SCXOBJ_DIR)/,$(auto-test-targets)))
 
diff --git a/tools/testing/selftests/sched_ext/total_bw.c b/tools/testing/selftests/sched_ext/total_bw.c
new file mode 100644
index 0000000000000..740c90a6ceab8
--- /dev/null
+++ b/tools/testing/selftests/sched_ext/total_bw.c
@@ -0,0 +1,281 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test to verify that total_bw value remains consistent across all CPUs
+ * in different BPF program states.
+ *
+ * Copyright (C) 2025 Nvidia Corporation.
+ */
+#include <bpf/bpf.h>
+#include <errno.h>
+#include <pthread.h>
+#include <scx/common.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "minimal.bpf.skel.h"
+#include "scx_test.h"
+
+#define MAX_CPUS 512
+#define STRESS_DURATION_SEC 5
+
+struct total_bw_ctx {
+	struct minimal *skel;
+	long baseline_bw[MAX_CPUS];
+	int nr_cpus;
+};
+
+static void *cpu_stress_thread(void *arg)
+{
+	volatile int i;
+	time_t end_time = time(NULL) + STRESS_DURATION_SEC;
+
+	while (time(NULL) < end_time)
+		for (i = 0; i < 1000000; i++)
+			;
+
+	return NULL;
+}
+
+/*
+ * The first enqueue on a CPU causes the DL server to start, for that
+ * reason run stressor threads in the hopes it schedules on all CPUs.
+ */
+static int run_cpu_stress(int nr_cpus)
+{
+	pthread_t *threads;
+	int i, ret = 0;
+
+	threads = calloc(nr_cpus, sizeof(pthread_t));
+	if (!threads)
+		return -ENOMEM;
+
+	/* Create threads to run on each CPU */
+	for (i = 0; i < nr_cpus; i++) {
+		if (pthread_create(&threads[i], NULL, cpu_stress_thread, NULL)) {
+			ret = -errno;
+			fprintf(stderr, "Failed to create thread %d: %s\n", i, strerror(-ret));
+			break;
+		}
+	}
+
+	/* Wait for all threads to complete */
+	for (i = 0; i < nr_cpus; i++) {
+		if (threads[i])
+			pthread_join(threads[i], NULL);
+	}
+
+	free(threads);
+	return ret;
+}
+
+static int read_total_bw_values(long *bw_values, int max_cpus)
+{
+	FILE *fp;
+	char line[256];
+	int cpu_count = 0;
+
+	fp = fopen("/sys/kernel/debug/sched/debug", "r");
+	if (!fp) {
+		SCX_ERR("Failed to open debug file");
+		return -1;
+	}
+
+	while (fgets(line, sizeof(line), fp)) {
+		char *bw_str = strstr(line, "total_bw");
+
+		if (bw_str) {
+			bw_str = strchr(bw_str, ':');
+			if (bw_str) {
+				/* Only store up to max_cpus values */
+				if (cpu_count < max_cpus)
+					bw_values[cpu_count] = atol(bw_str + 1);
+				cpu_count++;
+			}
+		}
+	}
+
+	fclose(fp);
+	return cpu_count;
+}
+
+static bool verify_total_bw_consistency(long *bw_values, int count)
+{
+	int i;
+	long first_value;
+
+	if (count <= 0)
+		return false;
+
+	first_value = bw_values[0];
+
+	for (i = 1; i < count; i++) {
+		if (bw_values[i] != first_value) {
+			SCX_ERR("Inconsistent total_bw: CPU0=%ld, CPU%d=%ld",
+				first_value, i, bw_values[i]);
+			return false;
+		}
+	}
+
+	return true;
+}
+
+static int fetch_verify_total_bw(long *bw_values, int nr_cpus)
+{
+	int attempts = 0;
+	int max_attempts = 10;
+	int count;
+
+	/*
+	 * The first enqueue on a CPU causes the DL server to start, for that
+	 * reason run stressor threads in the hopes it schedules on all CPUs.
+	 */
+	if (run_cpu_stress(nr_cpus) < 0) {
+		SCX_ERR("Failed to run CPU stress");
+		return -1;
+	}
+
+	/* Try multiple times to get stable values */
+	while (attempts < max_attempts) {
+		count = read_total_bw_values(bw_values, nr_cpus);
+		fprintf(stderr, "Read %d total_bw values (testing %d CPUs)\n", count, nr_cpus);
+		/* If system has more CPUs than we're testing, that's OK */
+		if (count < nr_cpus) {
+			SCX_ERR("Expected at least %d CPUs, got %d", nr_cpus, count);
+			attempts++;
+			sleep(1);
+			continue;
+		}
+
+		/* Only verify the CPUs we're testing */
+		if (verify_total_bw_consistency(bw_values, nr_cpus)) {
+			fprintf(stderr, "Values are consistent: %ld\n", bw_values[0]);
+			return 0;
+		}
+
+		attempts++;
+		sleep(1);
+	}
+
+	return -1;
+}
+
+static enum scx_test_status setup(void **ctx)
+{
+	struct total_bw_ctx *test_ctx;
+
+	if (access("/sys/kernel/debug/sched/debug", R_OK) != 0) {
+		fprintf(stderr, "Skipping test: debugfs sched/debug not accessible\n");
+		return SCX_TEST_SKIP;
+	}
+
+	test_ctx = calloc(1, sizeof(*test_ctx));
+	if (!test_ctx)
+		return SCX_TEST_FAIL;
+
+	test_ctx->nr_cpus = sysconf(_SC_NPROCESSORS_ONLN);
+	if (test_ctx->nr_cpus <= 0) {
+		free(test_ctx);
+		return SCX_TEST_FAIL;
+	}
+
+	/* If system has more CPUs than MAX_CPUS, just test the first MAX_CPUS */
+	if (test_ctx->nr_cpus > MAX_CPUS)
+		test_ctx->nr_cpus = MAX_CPUS;
+
+	/* Test scenario 1: BPF program not loaded */
+	/* Read and verify baseline total_bw before loading BPF program */
+	fprintf(stderr, "BPF prog initially not loaded, reading total_bw values\n");
+	if (fetch_verify_total_bw(test_ctx->baseline_bw, test_ctx->nr_cpus) < 0) {
+		SCX_ERR("Failed to get stable baseline values");
+		free(test_ctx);
+		return SCX_TEST_FAIL;
+	}
+
+	/* Load the BPF skeleton */
+	test_ctx->skel = minimal__open();
+	if (!test_ctx->skel) {
+		free(test_ctx);
+		return SCX_TEST_FAIL;
+	}
+
+	SCX_ENUM_INIT(test_ctx->skel);
+	if (minimal__load(test_ctx->skel)) {
+		minimal__destroy(test_ctx->skel);
+		free(test_ctx);
+		return SCX_TEST_FAIL;
+	}
+
+	*ctx = test_ctx;
+	return SCX_TEST_PASS;
+}
+
+static enum scx_test_status run(void *ctx)
+{
+	struct total_bw_ctx *test_ctx = ctx;
+	struct bpf_link *link;
+	long loaded_bw[MAX_CPUS];
+	long unloaded_bw[MAX_CPUS];
+	int i;
+
+	/* Test scenario 2: BPF program loaded */
+	link = bpf_map__attach_struct_ops(test_ctx->skel->maps.minimal_ops);
+	if (!link) {
+		SCX_ERR("Failed to attach scheduler");
+		return SCX_TEST_FAIL;
+	}
+
+	fprintf(stderr, "BPF program loaded, reading total_bw values\n");
+	if (fetch_verify_total_bw(loaded_bw, test_ctx->nr_cpus) < 0) {
+		SCX_ERR("Failed to get stable values with BPF loaded");
+		bpf_link__destroy(link);
+		return SCX_TEST_FAIL;
+	}
+	bpf_link__destroy(link);
+
+	/* Test scenario 3: BPF program unloaded */
+	fprintf(stderr, "BPF program unloaded, reading total_bw values\n");
+	if (fetch_verify_total_bw(unloaded_bw, test_ctx->nr_cpus) < 0) {
+		SCX_ERR("Failed to get stable values after BPF unload");
+		return SCX_TEST_FAIL;
+	}
+
+	/* Verify all three scenarios have the same total_bw values */
+	for (i = 0; i < test_ctx->nr_cpus; i++) {
+		if (test_ctx->baseline_bw[i] != loaded_bw[i]) {
+			SCX_ERR("CPU%d: baseline_bw=%ld != loaded_bw=%ld",
+				i, test_ctx->baseline_bw[i], loaded_bw[i]);
+			return SCX_TEST_FAIL;
+		}
+
+		if (test_ctx->baseline_bw[i] != unloaded_bw[i]) {
+			SCX_ERR("CPU%d: baseline_bw=%ld != unloaded_bw=%ld",
+				i, test_ctx->baseline_bw[i], unloaded_bw[i]);
+			return SCX_TEST_FAIL;
+		}
+	}
+
+	fprintf(stderr, "All total_bw values are consistent across all scenarios\n");
+	return SCX_TEST_PASS;
+}
+
+static void cleanup(void *ctx)
+{
+	struct total_bw_ctx *test_ctx = ctx;
+
+	if (test_ctx) {
+		if (test_ctx->skel)
+			minimal__destroy(test_ctx->skel);
+		free(test_ctx);
+	}
+}
+
+struct scx_test total_bw = {
+	.name = "total_bw",
+	.description = "Verify total_bw consistency across BPF program states",
+	.setup = setup,
+	.run = run,
+	.cleanup = cleanup,
+};
+REGISTER_SCX_TEST(&total_bw)
-- 
2.51.0