Re: [PATCH v2] sched/stats: Fix run_delay over-count for migrated sched_delayed tasks

[K Prateek Nayak]

Hello Wei,

On 9/20/2026 10:01 AM, albin_yang@xxxxxxx wrote:
> From: Wei Yang <albinwyang@xxxxxxxxxxx>
> 
> With DELAY_DEQUEUE, a blocked task stays on the runqueue with
> se.sched_delayed set and its sched_info.last_queued is cleared, so the sleep
> is not counted into run_delay.
> 
> When such a delayed (sleeping) task is migrated across CPUs via the plain
> migration paths (move_queued_task / move_queued_task_locked, the latter used
> by __migrate_swap_task), activate_task(dst, 0) calls enqueue_task() without
> ENQUEUE_RESTORE, re-arming last_queued to the migration timestamp while the
> task is still sleeping. The later real wakeup (ENQUEUE_DELAYED) tries to
> re-arm last_queued at wakeup time but is suppressed because last_queued is
> already non-zero, so sched_info_arrive() folds the whole sleep duration
> between migration and wakeup into run_delay.
> 
> Load balance is affected too: the sched_delayed check in can_migrate_task()
> bails out only when env->migration_type != migrate_load, so it does not
> block migration when the type is migrate_load - which active load balance
> always uses (its lb_env leaves migration_type at 0 == migrate_load), and
> which regular load balance can also use via calculate_imbalance(). Either
> way the re-attach goes through attach_task() -> activate_task(rq, p,
> ENQUEUE_NOCLOCK), without ENQUEUE_RESTORE.
> 
> Fix by not re-arming last_queued for a sched_delayed task in
> sched_info_enqueue(). The wakeup path clears sched_delayed before reaching
> sched_info_enqueue(), so it still re-arms at the real wakeup time. Plain
> runnable tasks are unaffected.
> 
> Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
> Reported-by: MingTao Huang <mintaohuang@xxxxxxxxxxx>
> Signed-off-by: Wei Yang <albinwyang@xxxxxxxxxxx>
> Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>

Feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>

-- 
Thanks and Regards,
Prateek