[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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Inserting SCX_CPU_PREEMPT_FAIR shifts the value of SCX_CPU_PREEMPT_UNKNOWN without updating tools/sched_ext autogenerated headers, breaking feature detection for BPF schedulers.
--

commit 19f50c7bbf7f31acfd835724319513f85b45249d
Author: Cui Jian <cjian720@163.com>

sched_ext:SCX is preemptible by the Fair scheduler

This commit adds the SCX_CPU_PREEMPT_FAIR enum value to distinguish
preemptions caused by the Fair scheduler. It updates the core sched_ext
code to report this reason when the next scheduled task belongs to the
fair_sched_class.

> diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
> index a075732d4430d..3ce07bec58a55 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,

[Severity: Medium]
Does inserting SCX_CPU_PREEMPT_FAIR here shift the value of
SCX_CPU_PREEMPT_UNKNOWN and potentially break existing BPF programs that rely
on its previous value or use it as a maximum for array sizing?

Also, should tools/sched_ext/include/scx/enum_defs.autogen.h be updated to
include #define HAVE_SCX_CPU_PREEMPT_FAIR? Without it, BPF schedulers might
fail to detect this new feature via #ifdef, potentially mishandling the new
enum value.

>  	/* unknown reason for SCX being preempted */
>  	SCX_CPU_PREEMPT_UNKNOWN,
>  };

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260612175113.110447-1-cjian720@163.com?part=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

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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The patch adds `SCX_CPU_PREEMPT_FAIR` to `enum scx_cpu_preempt_reason` in `kernel/sched/ext_internal.h`, but fails to update the checked-in autogenerated BPF tool headers in `tools/sched_ext/include/scx/enum_defs.autogen.h`.
--

commit 0e9cc05c20430d666c8c1a1d7ea0741d9c28b2d3
Author: Cui Jian <cjian720@163.com>

sched_ext:SCX is preemptible by the Fair scheduler

This commit adds a new preemption reason, SCX_CPU_PREEMPT_FAIR, to distinguish
when a sched_ext task is preempted by the Fair scheduler. It updates the
internal enum and mapping function to improve observability and diagnostics.

> diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
> index a075732d4430d..3ce07bec58a55 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,

[Severity: Medium]
Should tools/sched_ext/include/scx/enum_defs.autogen.h be regenerated
and included in this patch?

When adding a new value to enum scx_cpu_preempt_reason, the autogenerated
BPF tool headers typically need updating so downstream BPF schedulers can
use feature detection macros like HAVE_SCX_CPU_PREEMPT_FAIR. 

Without this update, BPF applications cannot conditionally check for this
new preemption feature across different kernel versions.

>  	/* unknown reason for SCX being preempted */
>  	SCX_CPU_PREEMPT_UNKNOWN,
>  };

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260612161403.105775-1-cjian720@163.com?part=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

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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The autogenerated header enum_defs.autogen.h was not updated to include HAVE_SCX_CPU_PREEMPT_FAIR.
--

commit 6eebc3231d63fa92ff2758fc84fe12b421fbde6c
Author: Cui Jian <cjian720@163.com>

sched_ext:SCX is preemptible by the Fair scheduler

This commit adds a new SCX_CPU_PREEMPT_FAIR preempt reason for the sched_ext
subsystem. It updates the internal enum and preempt reason mapping to make it
easier to observe when a task is preempted by the Fair scheduler.

[ ... ]

> diff --git a/kernel/sched/ext_internal.h b/kernel/sched/ext_internal.h
> --- 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,

[Severity: Medium]
Should tools/sched_ext/include/scx/enum_defs.autogen.h also be updated in
this patch to include HAVE_SCX_CPU_PREEMPT_FAIR?

Without adding this to the autogenerated header, external BPF schedulers
cannot use #ifdef HAVE_SCX_CPU_PREEMPT_FAIR to safely compile and support
both old and new kernels.

>  	/* unknown reason for SCX being preempted */
>  	SCX_CPU_PREEMPT_UNKNOWN,
>  };

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260612152304.94821-1-cjian720@163.com?part=1