Re: [PATCH v14 09/13] sched/debug: Add migration stats due to non preferred CPUs

[Nathan Chancellor <nathan@kernel.org>]

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@linux.ibm.com>
...
> 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

-- 
Cheers,
Nathan