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

[Johannes Weiner]

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.