Re: [PATCH] sched/cache: Keep nr_pref_llc_running in the runnable domain

[Zhan Xusheng <zhanxusheng1024@gmail.com>]

From: Zhan Xusheng <zhanxusheng@xiaomi.com>

The ordering the skip depends on is not visible in the diff, so for the
record: dequeue_hierarchy() at fair.c:8148 reaches account_llc_dequeue()
through dequeue_entity(), and clear_delayed() runs only afterwards at
8164, so se->sched_delayed is still set when account_llc_dequeue() tests
it.

The sequences balance:

  enqueue, dequeue                   +1, -1
  enqueue, sleep, wake, dequeue      +1, -1 set_delayed,
                                     +1 clear_delayed, -1
  enqueue, sleep, real dequeue       +1, -1, skip, no re-add
  load balance moves a delayed task  source unchanged, destination +1
                                     with sched_delayed already clear

requeue_delayed_entity() does not reach account_entity_enqueue(), doing
only __enqueue_entity() and clear_delayed(), so the wake path cannot
double count.  account_mm_sched() cannot see a delayed task either, since
task_running_on_cpu() at fair.c:12245 requires task_on_rq_queued() on the
current cpu and the caller is update_curr().

sd->llc_counts is right to stay on queued semantics: both readers compare
it against another llc_counts value, at fair.c:11895 and 13228, so delayed
tasks shift both sides alike.

Reviewed-by: Zhan Xusheng <zhanxusheng@xiaomi.com>

Thanks,
Zhan Xusheng