[PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Cheng-Yang Chou]

scx_central only fast-pathed per-cpu kthreads (PF_KTHREAD &&
nr_cpus_allowed == 1) directly to the local DSQ, leaving non-kthread
tasks pinned to a single CPU to go through the central queue. When
dispatch_to_cpu() later dequeued such a task and found it couldn't run
on the current CPU, it could trigger scx_error() and disable the
scheduler:

  sched_ext: central: SCX_DSQ_LOCAL[_ON] cannot move migration disabled qemu-system-x86[223212] from CPU 1 to 0

Though scx_central is a demo scheduler and meant to dispatch all tasks
from a central CPU, it should remain robust under any circumstances.

Extend the handling to all tasks with nr_cpus_allowed == 1:
- select_cpu() and enqueue(): return the task's current CPU and dispatch
  directly to local DSQ for pinned tasks.
- dispatch_to_cpu(): use SCX_KICK_PREEMPT when kicking a task's home
  CPU after bouncing to the fallback DSQ, so the CPU preempts
  immediately rather than waiting for the next timer tick.

Test plan:
- run scx_central on the host, then launch virtme-ng in a separate
  terminal to create pinned tasks (qemu-system-x86), and verify stable
  operation.

Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
---
 tools/sched_ext/scx_central.bpf.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c
index 4efcce099bd5..5a02669483cc 100644
--- a/tools/sched_ext/scx_central.bpf.c
+++ b/tools/sched_ext/scx_central.bpf.c
@@ -96,7 +96,13 @@ s32 BPF_STRUCT_OPS(central_select_cpu, struct task_struct *p,
 	 * disturbing other CPUs. It's safe to blindly return the central cpu as
 	 * select_cpu() is a hint and if @p can't be on it, the kernel will
 	 * automatically pick a fallback CPU.
+	 *
+	 * For tasks pinned to a single CPU, return the current CPU to avoid
+	 * an unnecessary fallback.
 	 */
+	if (p->nr_cpus_allowed == 1)
+		return scx_bpf_task_cpu(p);
+
 	return central_cpu;
 }
 
@@ -107,12 +113,13 @@ void BPF_STRUCT_OPS(central_enqueue, struct task_struct *p, u64 enq_flags)
 	__sync_fetch_and_add(&nr_total, 1);
 
 	/*
-	 * Push per-cpu kthreads at the head of local dsq's and preempt the
-	 * corresponding CPU. This ensures that e.g. ksoftirqd isn't blocked
-	 * behind other threads which is necessary for forward progress
-	 * guarantee as we depend on the BPF timer which may run from ksoftirqd.
+	 * Push tasks pinned to a single CPU at the head of local dsq's and
+	 * preempt the corresponding CPU. This ensures that e.g. ksoftirqd
+	 * isn't blocked behind other threads which is necessary for forward
+	 * progress guarantee as we depend on the BPF timer which may run
+	 * from ksoftirqd.
 	 */
-	if ((p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1) {
+	if (p->nr_cpus_allowed == 1) {
 		__sync_fetch_and_add(&nr_locals, 1);
 		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_INF,
 				   enq_flags | SCX_ENQ_PREEMPT);
@@ -155,6 +162,13 @@ static bool dispatch_to_cpu(s32 cpu)
 		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
 			__sync_fetch_and_add(&nr_mismatches, 1);
 			scx_bpf_dsq_insert(p, FALLBACK_DSQ_ID, SCX_SLICE_INF, 0);
+
+			/*
+			 * Kick the task's home CPU to pick it up from the
+			 * fallback DSQ promptly.
+			 */
+			scx_bpf_kick_cpu(scx_bpf_task_cpu(p), SCX_KICK_PREEMPT);
+
 			bpf_task_release(p);
 			/*
 			 * We might run out of dispatch buffer slots if we continue dispatching
-- 
2.48.1

Re: [PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Kuba Piecuch]

Hi Cheng-Yang,

On Mon Apr 13, 2026 at 11:04 AM UTC, Cheng-Yang Chou wrote:
> @@ -107,12 +113,13 @@ void BPF_STRUCT_OPS(central_enqueue, struct task_struct *p, u64 enq_flags)
>  	__sync_fetch_and_add(&nr_total, 1);
>  
>  	/*
> -	 * Push per-cpu kthreads at the head of local dsq's and preempt the
> -	 * corresponding CPU. This ensures that e.g. ksoftirqd isn't blocked
> -	 * behind other threads which is necessary for forward progress
> -	 * guarantee as we depend on the BPF timer which may run from ksoftirqd.
> +	 * Push tasks pinned to a single CPU at the head of local dsq's and
> +	 * preempt the corresponding CPU. This ensures that e.g. ksoftirqd
> +	 * isn't blocked behind other threads which is necessary for forward
> +	 * progress guarantee as we depend on the BPF timer which may run
> +	 * from ksoftirqd.
>  	 */
> -	if ((p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1) {
> +	if (p->nr_cpus_allowed == 1) {

If you want to cover all cases, then I think you should also check
is_migration_disabled(p). When a task disables migration, it is not reflected
in p->cpus_ptr or p->nr_cpus_allowed until the task is switched out (see
migrate_disable_switch()), which happens __after__ ops.enqueue().
So here, you could see p->nr_cpus_allowed > 1 even though the task has
migration disabled.

Thanks,
Kuba

Re: [PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Cheng-Yang Chou]

[-- Attachment #1: Type: text/plain, Size: 665 bytes --]

Hi Kuba,

On Mon, Apr 13, 2026 at 03:29:37PM +0000, Kuba Piecuch wrote:
> > -	if ((p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1) {
> > +	if (p->nr_cpus_allowed == 1) {
> 
> If you want to cover all cases, then I think you should also check
> is_migration_disabled(p). When a task disables migration, it is not reflected
> in p->cpus_ptr or p->nr_cpus_allowed until the task is switched out (see
> migrate_disable_switch()), which happens __after__ ops.enqueue().
> So here, you could see p->nr_cpus_allowed > 1 even though the task has
> migration disabled.

Thanks for the feedback! 
I've attached the v2 patch with the updated logic.

-- 
Thanks,
Cheng-Yang

[-- Attachment #2: 0001-tools-sched_ext-Handle-pinned-tasks-in-scx_central.patch --]
[-- Type: text/x-diff, Size: 4247 bytes --]

From 038e42a0a2c6d0b23e6914eaafcb0de301b70258 Mon Sep 17 00:00:00 2001
From: Cheng-Yang Chou <yphbchou0911@gmail.com>
Date: Mon, 13 Apr 2026 15:19:59 +0800
Subject: [PATCH v2 sched_ext/for-7.1-fixes] tools/sched_ext: Handle pinned tasks in scx_central

scx_central only fast-pathed per-cpu kthreads (PF_KTHREAD &&
nr_cpus_allowed == 1) directly to the local DSQ, leaving non-kthread
tasks pinned to a single CPU to go through the central queue. When
dispatch_to_cpu() later dequeued such a task and found it couldn't run
on the current CPU, it could trigger scx_error() and disable the
scheduler:

  sched_ext: central: SCX_DSQ_LOCAL[_ON] cannot move migration disabled qemu-system-x86[223212] from CPU 1 to 0

Though scx_central is a demo scheduler and meant to dispatch all tasks
from a central CPU, it should remain robust under any circumstances.

Extend the handling to all tasks with nr_cpus_allowed == 1:
- select_cpu() and enqueue(): bypass the central queue for pinned and
  migration-disabled tasks. enqueue() also checks is_migration_disabled()
  directly because p->nr_cpus_allowed lags behind migrate_disable_switch().
- dispatch_to_cpu(): use SCX_KICK_PREEMPT when bouncing to the fallback
  DSQ so the home CPU preempts immediately.

Test plan:
- run scx_central on the host, then launch virtme-ng in a separate
  terminal to create pinned tasks (qemu-system-x86), and verify stable
  operation.

Suggested-by: Kuba Piecuch <jpiecuch@google.com>
Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
---
v2: Add is_migration_disabled() check in enqueue() as p->nr_cpus_allowed
    is not updated until migrate_disable_switch() runs during context
    switch, which happens after ops.enqueue(). (Kuba Piecuch)

 tools/sched_ext/scx_central.bpf.c | 28 +++++++++++++++++++++++-----
 1 file changed, 23 insertions(+), 5 deletions(-)

diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c
index 4efcce099bd5..67b160a7ce18 100644
--- a/tools/sched_ext/scx_central.bpf.c
+++ b/tools/sched_ext/scx_central.bpf.c
@@ -96,7 +96,13 @@ s32 BPF_STRUCT_OPS(central_select_cpu, struct task_struct *p,
 	 * disturbing other CPUs. It's safe to blindly return the central cpu as
 	 * select_cpu() is a hint and if @p can't be on it, the kernel will
 	 * automatically pick a fallback CPU.
+	 *
+	 * For tasks pinned to a single CPU, return the current CPU to avoid
+	 * an unnecessary fallback.
 	 */
+	if (p->nr_cpus_allowed == 1)
+		return scx_bpf_task_cpu(p);
+
 	return central_cpu;
 }
 
@@ -107,12 +113,17 @@ void BPF_STRUCT_OPS(central_enqueue, struct task_struct *p, u64 enq_flags)
 	__sync_fetch_and_add(&nr_total, 1);
 
 	/*
-	 * Push per-cpu kthreads at the head of local dsq's and preempt the
-	 * corresponding CPU. This ensures that e.g. ksoftirqd isn't blocked
-	 * behind other threads which is necessary for forward progress
-	 * guarantee as we depend on the BPF timer which may run from ksoftirqd.
+	 * Push tasks pinned to a single CPU or with migration disabled at
+	 * the head of local dsq's and preempt the corresponding CPU. This
+	 * ensures that e.g. ksoftirqd isn't blocked behind other threads
+	 * which is necessary for forward progress guarantee as we depend on
+	 * the BPF timer which may run from ksoftirqd.
+	 *
+	 * p->nr_cpus_allowed is not updated until migrate_disable_switch()
+	 * runs during context switch, which happens after ops.enqueue(). Check
+	 * is_migration_disabled() directly to catch tasks in this window.
 	 */
-	if ((p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1) {
+	if (p->nr_cpus_allowed == 1 || is_migration_disabled(p)) {
 		__sync_fetch_and_add(&nr_locals, 1);
 		scx_bpf_dsq_insert(p, SCX_DSQ_LOCAL, SCX_SLICE_INF,
 				   enq_flags | SCX_ENQ_PREEMPT);
@@ -155,6 +166,13 @@ static bool dispatch_to_cpu(s32 cpu)
 		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
 			__sync_fetch_and_add(&nr_mismatches, 1);
 			scx_bpf_dsq_insert(p, FALLBACK_DSQ_ID, SCX_SLICE_INF, 0);
+
+			/*
+			 * Kick the task's home CPU to pick it up from the
+			 * fallback DSQ promptly.
+			 */
+			scx_bpf_kick_cpu(scx_bpf_task_cpu(p), SCX_KICK_PREEMPT);
+
 			bpf_task_release(p);
 			/*
 			 * We might run out of dispatch buffer slots if we continue dispatching
-- 
2.48.1

Re: [PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Tejun Heo]

Hello,

On Tue, Apr 14, 2026 at 11:45:59AM +0800, Cheng-Yang Chou wrote:
> diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c
> index 4efcce099bd5..67b160a7ce18 100644
> --- a/tools/sched_ext/scx_central.bpf.c
> +++ b/tools/sched_ext/scx_central.bpf.c
> @@ -96,7 +96,13 @@ s32 BPF_STRUCT_OPS(central_select_cpu, struct task_struct *p,
>  	 * disturbing other CPUs. It's safe to blindly return the central cpu as
>  	 * select_cpu() is a hint and if @p can't be on it, the kernel will
>  	 * automatically pick a fallback CPU.
> +	 *
> +	 * For tasks pinned to a single CPU, return the current CPU to avoid
> +	 * an unnecessary fallback.
>  	 */
> +	if (p->nr_cpus_allowed == 1)
> +		return scx_bpf_task_cpu(p);
> +

This isn't necessary. select_cpu() isn't called if nr_cpus_allowed == 1.

> @@ -107,12 +113,17 @@ void BPF_STRUCT_OPS(central_enqueue, struct task_struct *p, u64 enq_flags)
>  	__sync_fetch_and_add(&nr_total, 1);
>  
>  	/*
> -	 * Push per-cpu kthreads at the head of local dsq's and preempt the
> -	 * corresponding CPU. This ensures that e.g. ksoftirqd isn't blocked
> -	 * behind other threads which is necessary for forward progress
> -	 * guarantee as we depend on the BPF timer which may run from ksoftirqd.
> +	 * Push tasks pinned to a single CPU or with migration disabled at
> +	 * the head of local dsq's and preempt the corresponding CPU. This
> +	 * ensures that e.g. ksoftirqd isn't blocked behind other threads
> +	 * which is necessary for forward progress guarantee as we depend on
> +	 * the BPF timer which may run from ksoftirqd.
> +	 *
> +	 * p->nr_cpus_allowed is not updated until migrate_disable_switch()
> +	 * runs during context switch, which happens after ops.enqueue(). Check
> +	 * is_migration_disabled() directly to catch tasks in this window.
>  	 */
> -	if ((p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1) {
> +	if (p->nr_cpus_allowed == 1 || is_migration_disabled(p)) {

Let's not expand the condition here. This is to guarantee forward progress
of the scheduler itself.

> @@ -155,6 +166,13 @@ static bool dispatch_to_cpu(s32 cpu)
>  		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {

Instead, can you expand the condition above to "if this task can't run on
the target CPU for whatever reason"?

Thanks.

-- 
tejun

Re: [PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Cheng-Yang Chou]

[-- Attachment #1: Type: text/plain, Size: 904 bytes --]

Hi Tejun,

On Tue, Apr 14, 2026 at 07:19:04AM -1000, Tejun Heo wrote:
> Hello,
> 
> > +	if (p->nr_cpus_allowed == 1)
> > +		return scx_bpf_task_cpu(p);
> > +
> 
> This isn't necessary. select_cpu() isn't called if nr_cpus_allowed == 1.
> 

Ack.

> > @@ -107,12 +113,17 @@ void BPF_STRUCT_OPS(central_enqueue, struct task_struct *p, u64 enq_flags)
> >  	__sync_fetch_and_add(&nr_total, 1);
> >  
> > -	if ((p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1) {
> > +	if (p->nr_cpus_allowed == 1 || is_migration_disabled(p)) {
> 
> Let's not expand the condition here. This is to guarantee forward progress
> of the scheduler itself.

Ack.

> > @@ -155,6 +166,13 @@ static bool dispatch_to_cpu(s32 cpu)
> >  		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
> 
> Instead, can you expand the condition above to "if this task can't run on
> the target CPU for whatever reason"?

Done!
 
-- 
Thanks,
Cheng-Yang

[-- Attachment #2: v3-0001-tools-sched_ext-Handle-migration-disabled-tasks-i.patch --]
[-- Type: text/x-diff, Size: 2405 bytes --]

From df62cc372cb9fab7aa9b666a6a127d8248923dd8 Mon Sep 17 00:00:00 2001
From: Cheng-Yang Chou <yphbchou0911@gmail.com>
Date: Mon, 13 Apr 2026 15:19:59 +0800
Subject: [PATCH v3 sched_ext/for-7.1] tools/sched_ext: Handle migration-disabled tasks in
 scx_central

When a task calls migrate_disable(), p->cpus_ptr is not updated until
migrate_disable_switch() runs during context switch, so dispatch_to_cpu()
may dequeue such a task and dispatch it to a CPU it cannot run on.

Extend the mismatch check in dispatch_to_cpu() to also test
is_migration_disabled() alongside the cpumask check, so tasks in this
window are bounced to the fallback DSQ.

Suggested-by: Andrea Righi <arighi@nvidia.com>
Suggested-by: Tejun Heo <tj@kernel.org>
Suggested-by: Kuba Piecuch <jpiecuch@google.com>
Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
---
v3:
 - Drop the select_cpu() shortcut, revert enqueue() to the kthread-only
   forward-progress path, and expand the dispatch_to_cpu() mismatch
   check to cover is_migration_disabled() (Tejun Heo)
 - Drop kick_cpu() on mismatch (Andrea Righi)
 - Reword commit subject and message with the fix

v2:
 - Add is_migration_disabled() check in enqueue() as p->nr_cpus_allowed
   is not updated until migrate_disable_switch() runs during context
   switch, which happens after ops.enqueue(). (Kuba Piecuch)

 tools/sched_ext/scx_central.bpf.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c
index 4efcce099bd5..64dd60b3e922 100644
--- a/tools/sched_ext/scx_central.bpf.c
+++ b/tools/sched_ext/scx_central.bpf.c
@@ -149,10 +149,14 @@ static bool dispatch_to_cpu(s32 cpu)
 		}
 
 		/*
-		 * If we can't run the task at the top, do the dumb thing and
-		 * bounce it to the fallback dsq.
+		 * If we can't run the task at the top for whatever reason,
+		 * bounce it to the fallback dsq. Also check
+		 * is_migration_disabled() explicitly as p->cpus_ptr may not
+		 * reflect the migration-disabled state yet if
+		 * migrate_disable_switch() hasn't run.
 		 */
-		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
+		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr) ||
+		    (is_migration_disabled(p) && scx_bpf_task_cpu(p) != cpu)) {
 			__sync_fetch_and_add(&nr_mismatches, 1);
 			scx_bpf_dsq_insert(p, FALLBACK_DSQ_ID, SCX_SLICE_INF, 0);
 			bpf_task_release(p);
-- 
2.48.1

Re: [PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Kuba Piecuch]

On Wed Apr 15, 2026 at 6:17 AM UTC, Cheng-Yang Chou wrote:
> From df62cc372cb9fab7aa9b666a6a127d8248923dd8 Mon Sep 17 00:00:00 2001
> From: Cheng-Yang Chou <yphbchou0911@gmail.com>
> Date: Mon, 13 Apr 2026 15:19:59 +0800
> Subject: [PATCH v3 sched_ext/for-7.1] tools/sched_ext: Handle migration-disabled tasks in
>  scx_central
>
> When a task calls migrate_disable(), p->cpus_ptr is not updated until
> migrate_disable_switch() runs during context switch, so dispatch_to_cpu()
> may dequeue such a task and dispatch it to a CPU it cannot run on.
>
> Extend the mismatch check in dispatch_to_cpu() to also test
> is_migration_disabled() alongside the cpumask check, so tasks in this
> window are bounced to the fallback DSQ.
>
> Suggested-by: Andrea Righi <arighi@nvidia.com>
> Suggested-by: Tejun Heo <tj@kernel.org>
> Suggested-by: Kuba Piecuch <jpiecuch@google.com>
> Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
> ---
> v3:
>  - Drop the select_cpu() shortcut, revert enqueue() to the kthread-only
>    forward-progress path, and expand the dispatch_to_cpu() mismatch
>    check to cover is_migration_disabled() (Tejun Heo)
>  - Drop kick_cpu() on mismatch (Andrea Righi)
>  - Reword commit subject and message with the fix
>
> v2:
>  - Add is_migration_disabled() check in enqueue() as p->nr_cpus_allowed
>    is not updated until migrate_disable_switch() runs during context
>    switch, which happens after ops.enqueue(). (Kuba Piecuch)
>
>  tools/sched_ext/scx_central.bpf.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/tools/sched_ext/scx_central.bpf.c b/tools/sched_ext/scx_central.bpf.c
> index 4efcce099bd5..64dd60b3e922 100644
> --- a/tools/sched_ext/scx_central.bpf.c
> +++ b/tools/sched_ext/scx_central.bpf.c
> @@ -149,10 +149,14 @@ static bool dispatch_to_cpu(s32 cpu)
>  		}
>  
>  		/*
> -		 * If we can't run the task at the top, do the dumb thing and
> -		 * bounce it to the fallback dsq.
> +		 * If we can't run the task at the top for whatever reason,
> +		 * bounce it to the fallback dsq. Also check
> +		 * is_migration_disabled() explicitly as p->cpus_ptr may not
> +		 * reflect the migration-disabled state yet if
> +		 * migrate_disable_switch() hasn't run.
>  		 */
> -		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
> +		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr) ||
> +		    (is_migration_disabled(p) && scx_bpf_task_cpu(p) != cpu)) {
>  			__sync_fetch_and_add(&nr_mismatches, 1);
>  			scx_bpf_dsq_insert(p, FALLBACK_DSQ_ID, SCX_SLICE_INF, 0);
>  			bpf_task_release(p);

Looks good to me!

Minor email-related comment: I would prefer if patch revisions (v2, v3, ...)
were sent in separate emails with their own subject line, otherwise it may look
like I'm reviewing the v1, if you look solely at the subject line.

To be clear, the following tag applies to v3.

Reviewed-by: Kuba Piecuch <jpiecuch@google.com>

Re: [PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Cheng-Yang Chou]

Hi Kuba,

On Wed, Apr 15, 2026 at 09:59:33AM +0000, Kuba Piecuch wrote:
> Looks good to me!
> 
> Minor email-related comment: I would prefer if patch revisions (v2, v3, ...)
> were sent in separate emails with their own subject line, otherwise it may look
> like I'm reviewing the v1, if you look solely at the subject line.
> 
> To be clear, the following tag applies to v3.
> 
> Reviewed-by: Kuba Piecuch <jpiecuch@google.com>
> 

Ahh, thanks for the review and reminder!
I was just being lazy because I thought keeping it in a single thread
would mean I wouldn't have to add links to the changelog (but yes, I
should add them anyway). 

I'll send future revisions in separate threads. Thanks!

-- 
Thanks,
Cheng-Yang

Re: [PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Tejun Heo]

Hello,

> Cheng-Yang Chou (1):
>   tools/sched_ext: Handle migration-disabled tasks in scx_central

Applied to sched_ext/for-7.2.

Thanks.

--
tejun

Re: [PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Andrea Righi]

Hi Cheng-Yang,

On Tue, Apr 14, 2026 at 11:45:59AM +0800, Cheng-Yang Chou wrote:
...
> @@ -155,6 +166,13 @@ static bool dispatch_to_cpu(s32 cpu)
>  		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
>  			__sync_fetch_and_add(&nr_mismatches, 1);
>  			scx_bpf_dsq_insert(p, FALLBACK_DSQ_ID, SCX_SLICE_INF, 0);
> +
> +			/*
> +			 * Kick the task's home CPU to pick it up from the
> +			 * fallback DSQ promptly.
> +			 */
> +			scx_bpf_kick_cpu(scx_bpf_task_cpu(p), SCX_KICK_PREEMPT);
> +

You can also pass SCX_ENQ_PREEMPT to scx_bpf_dsq_insert() and avoid calling
scx_bpf_kick_cpu().

More in general, do we really need to preempt the CPU here? It'll be preempted
in the central_timerfn() handler later, right? If we preempt the CPU here we may
have lots of preemption events when many affinitized tasks are running (i.e.,
stress-ng --race-sched 0).

Thanks,
-Andrea

>  			bpf_task_release(p);
>  			/*
>  			 * We might run out of dispatch buffer slots if we continue dispatching
> -- 
> 2.48.1
> 

Re: [PATCH] tools/sched_ext: Handle pinned tasks in scx_central

[Cheng-Yang Chou]

Hi Andrea,

On Tue, Apr 14, 2026 at 11:01:00PM +0200, Andrea Righi wrote:
> Hi Cheng-Yang,
> 
> On Tue, Apr 14, 2026 at 11:45:59AM +0800, Cheng-Yang Chou wrote:
> ...
> > @@ -155,6 +166,13 @@ static bool dispatch_to_cpu(s32 cpu)
> >  		if (!bpf_cpumask_test_cpu(cpu, p->cpus_ptr)) {
> >  			__sync_fetch_and_add(&nr_mismatches, 1);
> >  			scx_bpf_dsq_insert(p, FALLBACK_DSQ_ID, SCX_SLICE_INF, 0);
> > +
> > +			/*
> > +			 * Kick the task's home CPU to pick it up from the
> > +			 * fallback DSQ promptly.
> > +			 */
> > +			scx_bpf_kick_cpu(scx_bpf_task_cpu(p), SCX_KICK_PREEMPT);
> > +
> 
> You can also pass SCX_ENQ_PREEMPT to scx_bpf_dsq_insert() and avoid calling
> scx_bpf_kick_cpu().
> 
> More in general, do we really need to preempt the CPU here? It'll be preempted
> in the central_timerfn() handler later, right? If we preempt the CPU here we may
> have lots of preemption events when many affinitized tasks are running (i.e.,
> stress-ng --race-sched 0).

Yeah, this is wrong :/ Thanks for your explanation!
Will fix this in the upcoming patch, enjoy your time in the UK!

-- 
Thanks,
Cheng-Yang