[PATCH] sched_ext:SCX is preemptible by the Fair scheduler

[PATCH] sched_ext:SCX is preemptible by the Fair scheduler

[Cui Jian]

Hi Andrea,

While this callback will eventually be deprecated, SCX is still a relatively new scheduling framework. Incomplete preemption reason enums make it difficult to analyze interactions between different schedulers, and filling this gap significantly improves observability for existing SCX workloads.

This patch does not modify core scheduling logic, introduces zero performance overhead, and adds negligible long-term maintenance cost.

sched_ext currently distinguishes preemption caused by STOP,
DL and RT scheduling classes, but preemption from the Fair
scheduler is reported as SCX_CPU_PREEMPT_UNKNOWN.

When analyzing SCX scheduling behavior, it is useful to know
whether a task was preempted by RT/DL classes or switched out
in favor of a Fair-class task. The lack of a dedicated reason
makes debugging and observability more difficult.

Add SCX_CPU_PREEMPT_FAIR and report it when the next scheduled
task belongs to fair_sched_class.

This change improves scheduling diagnostics and observability
without affecting scheduling behavior.

Signed-off-by: Cui Jian <cjian720@163.com>
---
 kernel/sched/ext.c          | 2 ++
 kernel/sched/ext_internal.h | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 5d2d19473a82..6e0281218197 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3015,6 +3015,8 @@ preempt_reason_from_class(const struct sched_class *class)
 		return SCX_CPU_PREEMPT_DL;
 	if (class == &rt_sched_class)
 		return SCX_CPU_PREEMPT_RT;
+	if (class == &fair_sched_class)
+		return SCX_CPU_PREEMPT_FAIR;
 	return SCX_CPU_PREEMPT_UNKNOWN;
 }
 
diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
index a075732d4430..3ce07bec58a5 100644
--- a/kernel/sched/ext_internal.h
+++ b/kernel/sched/ext_internal.h
@@ -241,6 +241,8 @@ enum scx_cpu_preempt_reason {
 	SCX_CPU_PREEMPT_DL,
 	/* next task is being scheduled by &sched_class_stop */
 	SCX_CPU_PREEMPT_STOP,
+	/* next task is being scheduled by &sched_class_fair*/
+	SCX_CPU_PREEMPT_FAIR,
 	/* unknown reason for SCX being preempted */
 	SCX_CPU_PREEMPT_UNKNOWN,
 };
-- 
2.34.1

Re: [PATCH] sched_ext:SCX is preemptible by the Fair scheduler

[Andrea Righi]

Hi Cui,

On Sat, Jun 13, 2026 at 01:51:13AM +0800, Cui Jian wrote:
> Hi Andrea,
> 
> While this callback will eventually be deprecated, SCX is still a relatively new scheduling framework. Incomplete preemption reason enums make it difficult to analyze interactions between different schedulers, and filling this gap significantly improves observability for existing SCX workloads.
> 
> This patch does not modify core scheduling logic, introduces zero performance overhead, and adds negligible long-term maintenance cost.

Please, don't re-send a new patch just for this, at least you should add a v2
with a proper changelog. But in this case the discussion can continue in the
previous email thread.

Thanks,
-Andrea

> 
> sched_ext currently distinguishes preemption caused by STOP,
> DL and RT scheduling classes, but preemption from the Fair
> scheduler is reported as SCX_CPU_PREEMPT_UNKNOWN.
> 
> When analyzing SCX scheduling behavior, it is useful to know
> whether a task was preempted by RT/DL classes or switched out
> in favor of a Fair-class task. The lack of a dedicated reason
> makes debugging and observability more difficult.
> 
> Add SCX_CPU_PREEMPT_FAIR and report it when the next scheduled
> task belongs to fair_sched_class.
> 
> This change improves scheduling diagnostics and observability
> without affecting scheduling behavior.
> 
> Signed-off-by: Cui Jian <cjian720@163.com>
> ---
>  kernel/sched/ext.c          | 2 ++
>  kernel/sched/ext_internal.h | 2 ++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 5d2d19473a82..6e0281218197 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3015,6 +3015,8 @@ preempt_reason_from_class(const struct sched_class *class)
>  		return SCX_CPU_PREEMPT_DL;
>  	if (class == &rt_sched_class)
>  		return SCX_CPU_PREEMPT_RT;
> +	if (class == &fair_sched_class)
> +		return SCX_CPU_PREEMPT_FAIR;
>  	return SCX_CPU_PREEMPT_UNKNOWN;
>  }
>  
> diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
> index a075732d4430..3ce07bec58a5 100644
> --- a/kernel/sched/ext_internal.h
> +++ b/kernel/sched/ext_internal.h
> @@ -241,6 +241,8 @@ enum scx_cpu_preempt_reason {
>  	SCX_CPU_PREEMPT_DL,
>  	/* next task is being scheduled by &sched_class_stop */
>  	SCX_CPU_PREEMPT_STOP,
> +	/* next task is being scheduled by &sched_class_fair*/
> +	SCX_CPU_PREEMPT_FAIR,
>  	/* unknown reason for SCX being preempted */
>  	SCX_CPU_PREEMPT_UNKNOWN,
>  };
> -- 
> 2.34.1
> 

Re: [PATCH] sched_ext:SCX is preemptible by the Fair scheduler

[Andrea Righi]

Hi Cui,

On Sat, Jun 13, 2026 at 12:14:03AM +0800, Cui Jian wrote:
> sched_ext currently distinguishes preemption caused by STOP,
> DL and RT scheduling classes, but preemption from the Fair
> scheduler is reported as SCX_CPU_PREEMPT_UNKNOWN.
> 
> When analyzing SCX scheduling behavior, it is useful to know
> whether a task was preempted by RT/DL classes or switched out
> in favor of a Fair-class task. The lack of a dedicated reason
> makes debugging and observability more difficult.
> 
> Add SCX_CPU_PREEMPT_FAIR and report it when the next scheduled
> task belongs to fair_sched_class.
> 
> This change improves scheduling diagnostics and observability
> without affecting scheduling behavior.

The preempt reason is only used in ops.cpu_release(), which is a deprecated API
and will be removed in the next kernel versions (see the pr_warn() in
validate_ops(), kernel/sched/ext.c):

 pr_warn("ops->cpu_acquire/release() are deprecated, use sched_switch TP instead\n");

So, your patch makes sense, but I'm not sure it's worth updating the API at this
point.

Thanks,
-Andrea

> 
> Signed-off-by: Cui Jian <cjian720@163.com>
> ---
>  kernel/sched/ext.c          | 2 ++
>  kernel/sched/ext_internal.h | 2 ++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 5d2d19473a82..6e0281218197 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3015,6 +3015,8 @@ preempt_reason_from_class(const struct sched_class *class)
>  		return SCX_CPU_PREEMPT_DL;
>  	if (class == &rt_sched_class)
>  		return SCX_CPU_PREEMPT_RT;
> +	if (class == &fair_sched_class)
> +		return SCX_CPU_PREEMPT_FAIR;
>  	return SCX_CPU_PREEMPT_UNKNOWN;
>  }
>  
> diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
> index a075732d4430..3ce07bec58a5 100644
> --- a/kernel/sched/ext_internal.h
> +++ b/kernel/sched/ext_internal.h
> @@ -241,6 +241,8 @@ enum scx_cpu_preempt_reason {
>  	SCX_CPU_PREEMPT_DL,
>  	/* next task is being scheduled by &sched_class_stop */
>  	SCX_CPU_PREEMPT_STOP,
> +	/* next task is being scheduled by &sched_class_fair*/
> +	SCX_CPU_PREEMPT_FAIR,
>  	/* unknown reason for SCX being preempted */
>  	SCX_CPU_PREEMPT_UNKNOWN,
>  };
> -- 
> 2.34.1
> 

[PATCH] sched_ext:SCX is preemptible by the Fair scheduler

[Cui Jian]

sched_ext currently distinguishes preemption caused by STOP,
DL and RT scheduling classes, but preemption from the Fair
scheduler is reported as SCX_CPU_PREEMPT_UNKNOWN.

When analyzing SCX scheduling behavior, it is useful to know
whether a task was preempted by RT/DL classes or switched out
in favor of a Fair-class task. The lack of a dedicated reason
makes debugging and observability more difficult.

Add SCX_CPU_PREEMPT_FAIR and report it when the next scheduled
task belongs to fair_sched_class.

This change improves scheduling diagnostics and observability
without affecting scheduling behavior.

Signed-off-by: Cui Jian <cjian720@163.com>
---
 kernel/sched/ext.c          | 2 ++
 kernel/sched/ext_internal.h | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 5d2d19473a82..6e0281218197 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3015,6 +3015,8 @@ preempt_reason_from_class(const struct sched_class *class)
 		return SCX_CPU_PREEMPT_DL;
 	if (class == &rt_sched_class)
 		return SCX_CPU_PREEMPT_RT;
+	if (class == &fair_sched_class)
+		return SCX_CPU_PREEMPT_FAIR;
 	return SCX_CPU_PREEMPT_UNKNOWN;
 }
 
diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
index a075732d4430..3ce07bec58a5 100644
--- a/kernel/sched/ext_internal.h
+++ b/kernel/sched/ext_internal.h
@@ -241,6 +241,8 @@ enum scx_cpu_preempt_reason {
 	SCX_CPU_PREEMPT_DL,
 	/* next task is being scheduled by &sched_class_stop */
 	SCX_CPU_PREEMPT_STOP,
+	/* next task is being scheduled by &sched_class_fair*/
+	SCX_CPU_PREEMPT_FAIR,
 	/* unknown reason for SCX being preempted */
 	SCX_CPU_PREEMPT_UNKNOWN,
 };
-- 
2.34.1

[PATCH] sched_ext:SCX is preemptible by the Fair scheduler

[Cui Jian]

sched_ext currently distinguishes preemption caused by STOP,
DL and RT scheduling classes, but preemption from the Fair
scheduler is reported as SCX_CPU_PREEMPT_UNKNOWN.

When analyzing SCX scheduling behavior, it is useful to know
whether a task was preempted by RT/DL classes or switched out
in favor of a Fair-class task. The lack of a dedicated reason
makes debugging and observability more difficult.

Add SCX_CPU_PREEMPT_FAIR and report it when the next scheduled
task belongs to fair_sched_class.

This change improves scheduling diagnostics and observability
without affecting scheduling behavior.

Signed-off-by: Cui Jian <cjian720@163.com>
---
 kernel/sched/ext.c          | 2 ++
 kernel/sched/ext_internal.h | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 5d2d19473a82..6e0281218197 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3015,6 +3015,8 @@ preempt_reason_from_class(const struct sched_class *class)
 		return SCX_CPU_PREEMPT_DL;
 	if (class == &rt_sched_class)
 		return SCX_CPU_PREEMPT_RT;
+	if (class == &fair_sched_class)
+		return SCX_CPU_PREEMPT_FAIR;
 	return SCX_CPU_PREEMPT_UNKNOWN;
 }
 
diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
index a075732d4430..3ce07bec58a5 100644
--- a/kernel/sched/ext_internal.h
+++ b/kernel/sched/ext_internal.h
@@ -241,6 +241,8 @@ enum scx_cpu_preempt_reason {
 	SCX_CPU_PREEMPT_DL,
 	/* next task is being scheduled by &sched_class_stop */
 	SCX_CPU_PREEMPT_STOP,
+	/* next task is being scheduled by &sched_class_fair*/
+	SCX_CPU_PREEMPT_FAIR,
 	/* unknown reason for SCX being preempted */
 	SCX_CPU_PREEMPT_UNKNOWN,
 };
-- 
2.34.1