Re: [PATCH] sched/fair: Fix inaccurate h_nr_runnable accounting with delayed dequeue

[K Prateek Nayak]

Hello Vincent,

On 1/17/2025 6:55 PM, Vincent Guittot wrote:
> Hi Prateek,
>
> On Fri, 17 Jan 2025 at 11:59, K Prateek Nayak <kprateek.nayak@xxxxxxx> wrote:
> > set_delayed() adjusts cfs_rq->h_nr_runnable for the hierarchy when an
> > entity is delayed irrespective of whether the entity corresponds to a
> > task or a cfs_rq.
> >
> > Consider the following scenario:
> >
> >          root
> >         /    \
> >        A      B (*) delayed since B is no longer eligible on root
> >        |      |
> >      Task0  Task1 <--- dequeue_task_fair() - task blocks
> >
> > When Task1 blocks (dequeue_entity() for task's se returns true),
> > dequeue_entities() will continue adjusting cfs_rq->h_nr_* for the
> > hierarchy of Task1. However, when the sched_entity corresponding to
> > cfs_rq B is delayed, set_delayed() will adjust the h_nr_runnable for the
> > hierarchy too leading to both dequeue_entity() and set_delayed()
> > decrementing h_nr_runnable for the dequeue of the same task.
> >
> > A SCHED_WARN_ON() to inspect h_nr_runnable post its update in
> > dequeue_entities() like below:
> >
> >      cfs_rq->h_nr_runnable -= h_nr_runnable;
> >      SCHED_WARN_ON(((int) cfs_rq->h_nr_runnable) < 0);
> >
> > is consistently tripped when running wakeup intensive workloads like
> > hackbench in a cgroup.
> >
> > This error is self correcting since cfs_rq are per-cpu and cannot
> > migrate. The entitiy is either picked for full dequeue or is requeued
> > when a task wakes up below it. Both those paths call clear_delayed()
> > which again increments h_nr_runnable of the hierarchy without
> > considering if the entity corresponds to a task or not.
> >
> > h_nr_runnable will eventually reflect the correct value however in the
> > interim, the incorrect values can still influence PELT calculation which
> > uses se->runnable_weight or cfs_rq->h_nr_runnable.
> >
> > Since only delayed tasks take the early return path in
> > dequeue_entities() and enqueue_task_fair(), adjust the
> > h_nr_runnable in {set,clear}_delayed() only when a task is delayed as
> > this path skips the h_nr_* update loops and returns early.
> >
> > For entities corresponding to cfs_rq, the h_nr_* update loop in the
> > caller will do the right thing.
> >
> > Fixes: 76f2f783294d ("sched/eevdf: More PELT vs DELAYED_DEQUEUE")
>
> You probably mean  c2a295bffeaf ("sched/fair: Add new cfs_rq.h_nr_runnable")

You are right! I had done a git blame on set_delayed() ad landed at
commit 76f2f783294d but you are right, it should be c2a295bffeaf
("sched/fair: Add new cfs_rq.h_nr_runnable") when the accounting was
inverted to account runnable tasks. Thank you for pointing that out.

> Before we were tracking the opposite h_nr_delayed. Did you see the
> problem only on tip/sched/core or also before the rework which added
> h_nr_runnable and removed h_nr_delayed

The problem is on tip:sched/core. I did not encounter any anomalies on
76f2f783294d ("sched/eevdf: More PELT vs DELAYED_DEQUEUE")

"h_nr_delayed" was only adjusted in dequeue_entities() for "!seep &&
!delayed" which would imply migration or a save + restore type operation
and the whole "h_nr_delayed" adjusting was contained in
{set,clear}_delayed() for delayed dequeue, finish delayed dequeue, and
requeue.

> I'm going to have a closer look

Thank you!

> > Reviewed-by: Gautham R. Shenoy <gautham.shenoy@xxxxxxx>
> > Tested-by: Swapnil Sapkal <swapnil.sapkal@xxxxxxx>
> > Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> > ---
> >
> > [..snip..]

--
Thanks and Regards,
Prateek