Re: [PATCH v14 09/13] sched/debug: Add migration stats due to non preferred CPUs
From: Shrikanth Hegde
Date: Tue Sep 29 2026 - 11:00:03 EST
Hi Nathan.
On 9/29/26 6:13 PM, Shrikanth Hegde wrote:
Hi Nathan. Thanks for report.
On 9/29/26 5:48 PM, Nathan Chancellor wrote:
On Mon, Sep 28, 2026 at 11:07:24AM +0530, Shrikanth Hegde wrote:
Add a new per-task stat,...
- nr_migrations_cpu_non_preferred: number of push migrations while the
CPU is non-preferred.
Since this new stat is per-task, it changes only /proc/<pid>/sched.
It doesn't update /proc/schedstat. Hence increasing the schedstat version
is not necessary.
Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5049eff58fb7..7465c983e6f6 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -11236,8 +11236,13 @@ static int sched_non_preferred_cpu_push_stop(void *arg)
update_rq_clock(rq);
context_unsafe_alias(rq);
- if (task_rq(p) == rq && task_on_rq_queued(p))
- rq = __migrate_task(rq, &rf, p, cpu);
+ if (task_rq(p) == rq && task_on_rq_queued(p)) {
+ struct rq *dest_rq = __migrate_task(rq, &rf, p, cpu);
+
+ if (rq != dest_rq)
+ schedstat_inc(p->stats.nr_migrations_cpu_non_preferred);
+ rq = dest_rq;
+ }
rq_unlock(rq, &rf);
}
This breaks the build for me with clang-23+ (which have context analysis
enabled by default), although it bisects to the final patch of the
series since this is under CONFIG_PREFERRED_CPU and it is not selected
until then.
kernel/sched/core.c:11292:25: error: calling function '__migrate_task' requires holding raw_spinlock 'rq_lockp(rq)' exclusively [-Werror,-Wthread-safety-analysis]
11292 | struct rq *dest_rq = __migrate_task(rq, &rf, p, cpu);
| ^
kernel/sched/core.c:11298:3: error: releasing raw_spinlock 'rq_lockp(rq)' that was not held [-Werror,-Wthread-safety-analysis]
11298 | rq_unlock(rq, &rf);
| ^
kernel/sched/core.c:11303:1: error: raw_spinlock 'rq_lockp(__this_rq())' is not held on every path through here [-Werror,-Wthread-safety-analysis]
11303 | }
| ^
kernel/sched/core.c:11286:3: note: raw_spinlock acquired here
11286 | rq_lock(rq, &rf);
| ^
3 errors generated.
Not sure what the proper fix for this is, maybe another
context_unsafe_alias()? cc Marco just in case
I suspect it is due to using of dest_rq = rq and rq is changing context.
let me try locally and see the fix.
One fix is use rq->cpu instead of rq comparison to see if migration happened.
Well, it was rather due to Patch 8/13 which had rq marked as context_unsafe_alias
after acquiring it.
I have tried below and that helps to fix the warnings.
I will write a changelog and send it across soon.
Let me know if it works for you.
---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0bb86a43a592..23677d76f9d2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -11283,10 +11283,10 @@ static int sched_non_preferred_cpu_push_stop(void *arg)
* safely bail out.
*/
cpu = select_fallback_rq(rq->cpu, p);
+ context_unsafe_alias(rq);
rq_lock(rq, &rf);
rq->npc_push_work_pending = false;
update_rq_clock(rq);
- context_unsafe_alias(rq);
if (task_rq(p) == rq && task_on_rq_queued(p)) {
struct rq *dest_rq = __migrate_task(rq, &rf, p, cpu);