Re: [REGRESSION] Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

[K Prateek Nayak]

Hello Johannes,

On 10/9/2024 11:37 PM, Johannes Weiner wrote:
> On Tue, Oct 08, 2024 at 09:54:52PM +0530, K Prateek Nayak wrote:
> >  From 2e15180e18b51e9a2bc0d7050e915a70d2673a06 Mon Sep 17 00:00:00 2001
> > From: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> > Date: Fri, 4 Oct 2024 15:24:35 +0000
> > Subject: [RFC PATCH] sched/psi: Fixup PSI accounting with DELAY_DEQUEUE
> >
> > After the merge of DELAY_DEQUEUE, "psi: inconsistent task state: warning
> > were seen early into the boot. The crux of the matter is the fact that
> > when a task is delayed, and the delayed task is then migrated, the
> > wakeup context may not have any idea that the task was moved from its
> > previous runqueue. This is the same reason psi_enqueue() considers
> > only ...
> >
> >       (flags & ENQUEUE_WAKEUP) && !(flags & ENQUEUE_MIGRATED)
> >
> > ... as a wakeup. In case of a wakeup with migration, PSI forgoes
> > clearing the TSK_IOWAIT flag which seems to be the issue I encountered
> > in my splat previously.
> >
> > With that said, the below diff, based on Peter's original approach
> > currently seems to work for me in the sense that I have not seen the
> > inconsistent state warning for a while now with my stress test.
> >
> > Two key points of the approach are:
> >
> > o It uses "p->migration_flags" to indicate a delayed entity has
> >     migrated to another runqueue and convey the same during psi_enqueue().
> >
> > o It adds ENQUEUE_WAKEUP flag alongside ENQUEUE_DELAYED for
> >     enqueue_task() in ttwu_runnable() since psi_enqueue() needs to know of
> >     a wakeup without migration to clear the TSK_IOWAIT flag it would have
> >     set during psi_task_switch() for blocking task and going down the
> >     stack for enqueue_task_fair(), there seem to be no other observer of
> >     the ENQUEUE_WAKEUP flag other than psi_enqueue() in the requeue path.
> >
> > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
>
> Tested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
>
> It fixes the warning and bogus pressure values after stressing it for
> an hour or so with tons of cpu contention and cgroup movements.

Thanks a ton for testing. I'll send a proper series soon for folks to
review.
--
Thanks and Regards,
Prateek