[PATCH] sched_ext: Print deprecation warnings only once

[PATCH] sched_ext: Print deprecation warnings only once

[Breno Leitao]

The deprecation notices for direct p->scx.slice/dsq_vtime writes and for
ops->cpu_acquire/release() use plain pr_warn(), so they repeat on every
scheduler (re)load and flood the kernel log.

The slice/dsq_vtime notice is emitted from the BPF verifier's
btf_struct_access callback, which runs once per write access and is
re-evaluated as the verifier explores paths, so loading a single
scheduler can print it several times. The cpu_acquire/release notice is
printed on every scheduler enable.

Switch both to pr_warn_once() so each deprecation is reported a single
time, and add the missing newline to the slice/dsq_vtime message.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 kernel/sched/ext/ext.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 691d53fe0f648..708eb9024cb25 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -6988,7 +6988,7 @@ static int validate_ops(struct scx_sched *sch, const struct sched_ext_ops *ops)
 	 * run past the BPF allocation. Skip for cid-form.
 	 */
 	if (!sch->is_cid_type && (ops->cpu_acquire || ops->cpu_release))
-		pr_warn("ops->cpu_acquire/release() are deprecated, use sched_switch TP instead\n");
+		pr_warn_once("ops->cpu_acquire/release() are deprecated, use sched_switch TP instead\n");
 
 	/*
 	 * Sub-scheduler support is tied to the cid-form struct_ops. A sub-sched
@@ -7806,7 +7806,7 @@ static int bpf_scx_btf_struct_access(struct bpf_verifier_log *log,
 		     off + size <= offsetofend(struct task_struct, scx.slice)) ||
 		    (off >= offsetof(struct task_struct, scx.dsq_vtime) &&
 		     off + size <= offsetofend(struct task_struct, scx.dsq_vtime))) {
-			pr_warn("sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()");
+			pr_warn_once("sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()\n");
 			return SCALAR_VALUE;
 		}
 

---
base-commit: 4e5dfb7c84012007c3c7061126491bbc92d71bf1
change-id: 20260623-scx_warning-0df49c442afb

Best regards,
-- 
Breno Leitao <leitao@debian.org>

Re: [PATCH] sched_ext: Print deprecation warnings only once

[Christian Loehle]

On 6/23/26 15:38, Breno Leitao wrote:
> The deprecation notices for direct p->scx.slice/dsq_vtime writes and for
> ops->cpu_acquire/release() use plain pr_warn(), so they repeat on every
> scheduler (re)load and flood the kernel log.
> > The slice/dsq_vtime notice is emitted from the BPF verifier's
> btf_struct_access callback, which runs once per write access and is
> re-evaluated as the verifier explores paths, so loading a single
> scheduler can print it several times. The cpu_acquire/release notice is
> printed on every scheduler enable.
> 
> Switch both to pr_warn_once() so each deprecation is reported a single
> time, and add the missing newline to the slice/dsq_vtime message.

I guess this should be warned for once per BPF scheduler like
sch->warned_deprecated_rq

> 
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
>  kernel/sched/ext/ext.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index 691d53fe0f648..708eb9024cb25 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -6988,7 +6988,7 @@ static int validate_ops(struct scx_sched *sch, const struct sched_ext_ops *ops)
>  	 * run past the BPF allocation. Skip for cid-form.
>  	 */
>  	if (!sch->is_cid_type && (ops->cpu_acquire || ops->cpu_release))
> -		pr_warn("ops->cpu_acquire/release() are deprecated, use sched_switch TP instead\n");
> +		pr_warn_once("ops->cpu_acquire/release() are deprecated, use sched_switch TP instead\n");
>  
>  	/*
>  	 * Sub-scheduler support is tied to the cid-form struct_ops. A sub-sched
> @@ -7806,7 +7806,7 @@ static int bpf_scx_btf_struct_access(struct bpf_verifier_log *log,
>  		     off + size <= offsetofend(struct task_struct, scx.slice)) ||
>  		    (off >= offsetof(struct task_struct, scx.dsq_vtime) &&
>  		     off + size <= offsetofend(struct task_struct, scx.dsq_vtime))) {
> -			pr_warn("sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()");
> +			pr_warn_once("sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()\n");
>  			return SCALAR_VALUE;
>  		}
>  
> 
> ---
> base-commit: 4e5dfb7c84012007c3c7061126491bbc92d71bf1
> change-id: 20260623-scx_warning-0df49c442afb
> 
> Best regards,

Re: [PATCH] sched_ext: Print deprecation warnings only once

[Andrea Righi]

Hi Breno and Christian,

On Tue, Jun 23, 2026 at 04:45:55PM +0100, Christian Loehle wrote:
> On 6/23/26 15:38, Breno Leitao wrote:
> > The deprecation notices for direct p->scx.slice/dsq_vtime writes and for
> > ops->cpu_acquire/release() use plain pr_warn(), so they repeat on every
> > scheduler (re)load and flood the kernel log.
> > > The slice/dsq_vtime notice is emitted from the BPF verifier's
> > btf_struct_access callback, which runs once per write access and is
> > re-evaluated as the verifier explores paths, so loading a single
> > scheduler can print it several times. The cpu_acquire/release notice is
> > printed on every scheduler enable.
> > 
> > Switch both to pr_warn_once() so each deprecation is reported a single
> > time, and add the missing newline to the slice/dsq_vtime message.
> 
> I guess this should be warned for once per BPF scheduler like
> sch->warned_deprecated_rq

Yeah, I agree. The thing is that we can load multiple different BPF schedulers,
and we'd want to get these messages for each of them. So ideally we need
something like sch->warned_deprecated_rq.

Thanks,
-Andrea

> 
> > 
> > Signed-off-by: Breno Leitao <leitao@debian.org>
> > ---
> >  kernel/sched/ext/ext.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> > index 691d53fe0f648..708eb9024cb25 100644
> > --- a/kernel/sched/ext/ext.c
> > +++ b/kernel/sched/ext/ext.c
> > @@ -6988,7 +6988,7 @@ static int validate_ops(struct scx_sched *sch, const struct sched_ext_ops *ops)
> >  	 * run past the BPF allocation. Skip for cid-form.
> >  	 */
> >  	if (!sch->is_cid_type && (ops->cpu_acquire || ops->cpu_release))
> > -		pr_warn("ops->cpu_acquire/release() are deprecated, use sched_switch TP instead\n");
> > +		pr_warn_once("ops->cpu_acquire/release() are deprecated, use sched_switch TP instead\n");
> >  
> >  	/*
> >  	 * Sub-scheduler support is tied to the cid-form struct_ops. A sub-sched
> > @@ -7806,7 +7806,7 @@ static int bpf_scx_btf_struct_access(struct bpf_verifier_log *log,
> >  		     off + size <= offsetofend(struct task_struct, scx.slice)) ||
> >  		    (off >= offsetof(struct task_struct, scx.dsq_vtime) &&
> >  		     off + size <= offsetofend(struct task_struct, scx.dsq_vtime))) {
> > -			pr_warn("sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()");
> > +			pr_warn_once("sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()\n");
> >  			return SCALAR_VALUE;
> >  		}
> >  
> > 
> > ---
> > base-commit: 4e5dfb7c84012007c3c7061126491bbc92d71bf1
> > change-id: 20260623-scx_warning-0df49c442afb
> > 
> > Best regards,
> 

Re: [PATCH] sched_ext: Print deprecation warnings only once

[Breno Leitao]

On Tue, Jun 23, 2026 at 05:53:33PM +0200, Andrea Righi wrote:
> Hi Breno and Christian,
> 
> On Tue, Jun 23, 2026 at 04:45:55PM +0100, Christian Loehle wrote:
> > On 6/23/26 15:38, Breno Leitao wrote:
> > > The deprecation notices for direct p->scx.slice/dsq_vtime writes and for
> > > ops->cpu_acquire/release() use plain pr_warn(), so they repeat on every
> > > scheduler (re)load and flood the kernel log.
> > > > The slice/dsq_vtime notice is emitted from the BPF verifier's
> > > btf_struct_access callback, which runs once per write access and is
> > > re-evaluated as the verifier explores paths, so loading a single
> > > scheduler can print it several times. The cpu_acquire/release notice is
> > > printed on every scheduler enable.
> > > 
> > > Switch both to pr_warn_once() so each deprecation is reported a single
> > > time, and add the missing newline to the slice/dsq_vtime message.
> > 
> > I guess this should be warned for once per BPF scheduler like
> > sch->warned_deprecated_rq
> 
> Yeah, I agree. The thing is that we can load multiple different BPF schedulers,
> and we'd want to get these messages for each of them. So ideally we need
> something like sch->warned_deprecated_rq.

Agreed, pr_warn_once() is too coarse - it mutes the notice for every
scheduler loaded after the first one since boot.

For ops->cpu_acquire/release() this is easy: the notice comes from
validate_ops(), which has the scx_sched, so I'll gate it on a
per-scheduler warned_deprecated_cpu_rel flag, just like
warned_deprecated_rq.

The slice/dsq_vtime notice is the one I wanted to ask about. It's
emitted from bpf_scx_btf_struct_access(), the .btf_struct_access
verifier callback.

That runs at program load/verification time, before the struct_ops is
registered and the scx_sched is allocated (that only happens later in
scx_enable()). Any idea on what to do in this case?

Thanks,
--breno

Re: [PATCH] sched_ext: Print deprecation warnings only once

[Andrea Righi]

Hi Breno,

On Tue, Jun 23, 2026 at 09:45:06AM -0700, Breno Leitao wrote:
> On Tue, Jun 23, 2026 at 05:53:33PM +0200, Andrea Righi wrote:
> > Hi Breno and Christian,
> > 
> > On Tue, Jun 23, 2026 at 04:45:55PM +0100, Christian Loehle wrote:
> > > On 6/23/26 15:38, Breno Leitao wrote:
> > > > The deprecation notices for direct p->scx.slice/dsq_vtime writes and for
> > > > ops->cpu_acquire/release() use plain pr_warn(), so they repeat on every
> > > > scheduler (re)load and flood the kernel log.
> > > > > The slice/dsq_vtime notice is emitted from the BPF verifier's
> > > > btf_struct_access callback, which runs once per write access and is
> > > > re-evaluated as the verifier explores paths, so loading a single
> > > > scheduler can print it several times. The cpu_acquire/release notice is
> > > > printed on every scheduler enable.
> > > > 
> > > > Switch both to pr_warn_once() so each deprecation is reported a single
> > > > time, and add the missing newline to the slice/dsq_vtime message.
> > > 
> > > I guess this should be warned for once per BPF scheduler like
> > > sch->warned_deprecated_rq
> > 
> > Yeah, I agree. The thing is that we can load multiple different BPF schedulers,
> > and we'd want to get these messages for each of them. So ideally we need
> > something like sch->warned_deprecated_rq.
> 
> Agreed, pr_warn_once() is too coarse - it mutes the notice for every
> scheduler loaded after the first one since boot.
> 
> For ops->cpu_acquire/release() this is easy: the notice comes from
> validate_ops(), which has the scx_sched, so I'll gate it on a
> per-scheduler warned_deprecated_cpu_rel flag, just like
> warned_deprecated_rq.
> 
> The slice/dsq_vtime notice is the one I wanted to ask about. It's
> emitted from bpf_scx_btf_struct_access(), the .btf_struct_access
> verifier callback.
> 
> That runs at program load/verification time, before the struct_ops is
> registered and the scx_sched is allocated (that only happens later in
> scx_enable()). Any idea on what to do in this case?

Hm... actually thinking more about it, the existing behavior seems fine already.

The ops->cpu_acquire/release() warning is only emitted from validate_ops(),
which runs when the scheduler is registered. It's not in the runtime callback
path, so we should already get one warning per scheduler registration. That
means we don't need a warned_deprecated_cpu_rel flag.

For slice/dsq_vtime, the warning is emitted when the verifier sees write
accesses to those fields. It can fire multiple times while loading one sched_ext
scheduler, depending on how many such accesses the verifier observes, but this
also seems correct.

If we want to prevent potential log spam maybe we can just change those
pr_warn() to pr_warn_ratelimited()?

Thanks,
-Andrea