Re: [PATCH] sched/proxy_exec: Optimize proxy_tag_curr() pushable removal

[K Prateek Nayak]

Hello Zhidao,

On 3/3/2026 5:27 PM, soolaugust@xxxxxxxxx wrote:
> The SAVE/RESTORE cycle achieves pushable removal only indirectly:
> enqueue_task_rt/dl() suppresses re-enqueue into the pushable
> list when task_is_blocked(owner) is true.  The same result is
> obtained more directly by calling dequeue_pushable_task() or
> dequeue_pushable_dl_task() once, without any of the side effects.
> 
> Replace the workaround with per-class direct calls:

I had this question in the past and from my reading, I think we
can safely call the dequeue_pushable*() helper (John might have some
notes on these bits) but that direct call is just horrible!

Why can't we just have a sched_class->proxy_tag_curr(rq, p) or such
and just call it if a "sched_class" populates it?

> 
>   RT: dequeue_pushable_task(rq, owner)   -- O(1) plist remove
>   DL: dequeue_pushable_dl_task(rq, owner) -- O(log n) rb_erase,
>       but avoids the bandwidth counter churn entirely
>   CFS: no-op (no pushable list; task_is_blocked() suffices)
> 
> Both functions are promoted from static and declared in sched.h.
> deadline.c also gains the missing isolation.h include required
> by dl_get_task_effective_cpus().
> 
> Signed-off-by: zhidao su <suzhidao@xxxxxxxxxx>
> ---
>  kernel/sched/core.c     | 28 +++++++++++++++++++---------
>  kernel/sched/deadline.c |  3 ++-
>  kernel/sched/rt.c       |  2 +-
>  kernel/sched/sched.h    |  2 ++
>  4 files changed, 24 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index dc9f17b35e4..2aba15d84b7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6728,16 +6728,26 @@ static inline void proxy_tag_curr(struct rq *rq, struct task_struct *owner)
>         if (!sched_proxy_exec())
>                 return;
>         /*
> -        * pick_next_task() calls set_next_task() on the chosen task
> -        * at some point, which ensures it is not push/pullable.
> -        * However, the chosen/donor task *and* the mutex owner form an
> -        * atomic pair wrt push/pull.
> +        * The donor goes through set_next_task() which calls
> +        * dequeue_pushable_task() making it non-pushable.  The owner
> +        * does not go through that path, so we must remove it from
> +        * the pushable list explicitly.
>          *
> -        * Make sure owner we run is not pushable. Unfortunately we can
> -        * only deal with that by means of a dequeue/enqueue cycle. :-/
> -        */
> -       dequeue_task(rq, owner, DEQUEUE_NOCLOCK | DEQUEUE_SAVE);
> -       enqueue_task(rq, owner, ENQUEUE_NOCLOCK | ENQUEUE_RESTORE);
> +        * For RT tasks: remove from the plist directly.
> +        * For DL tasks: remove from the rb-tree directly.
> +        * For CFS tasks: no pushable list exists; can_migrate_task()
> +        * already rejects blocked owners via task_is_blocked().

Tag is for the current running context (owwner). task_on_cpu() check is
what guards that for fair.

> +        *
> +        * The prior dequeue/enqueue(SAVE/RESTORE) cycle achieved the
> +        * same result by relying on task_is_blocked() suppressing the
> +        * re-enqueue into the pushable list, but it carried O(log n)
> +        * overhead and, for DL owners, triggered sub_running_bw() +
> +        * sub_rq_bw() -- bandwidth counter churn with no net effect.
> +        */
> +       if (rt_task(owner))
> +               dequeue_pushable_task(rq, owner);
> +       else if (dl_task(owner))
> +               dequeue_pushable_dl_task(rq, owner);

I think a sched_class callback would be better and it'll also allow
other sched classes to just populate it when need arises instead of
adding more else if here. Like:

  if (p->sched_class->proxy_tag_curr) /* or dequeue_pushable_task()? */
    p->sched_class->proxy_tag_curr(rq, p);

Also I forgot how the current context gets put back on the pushable list
once the proxy is done since at that point it is just a preempted task
on the CPU.

John, do you remember how this happens?

I think we might also need a proxy_untag_current() which calls into
enqueue_pushable.*_task() which does the minimal bits of put_prev_task()
undoing the tag and possibly queuing some balance callbacks which would
have been skipped by set_next_task() on not seeing any pushable tasks
at the time of donor pick?

>  }
> 
>  /*

-- 
Thanks and Regards,
Prateek