[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@xxxxxxx>
---
 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