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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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@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
> 

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.