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

[K Prateek Nayak <kprateek.nayak@amd.com>]

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@amd.com>
>> 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@infradead.org>
>> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
> 
> Tested-by: Johannes Weiner <hannes@cmpxchg.org>
> 
> 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