linux-kernel - Re: [PATCH 10/24] sched/uclamg: Handle delayed dequeue

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Message-ID: <156cfeac-bc29-47ef-a296-b523a619234e@arm.com>
Date: Wed, 11 Sep 2024 10:38:42 +0100
From: Luis Machado <luis.machado@....com>
To: Mike Galbraith <efault@....de>, Peter Zijlstra <peterz@...radead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Hongyan Xia <hongyan.xia2@....com>, mingo@...hat.com, juri.lelli@...hat.com,
 rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
 vschneid@...hat.com, linux-kernel@...r.kernel.org, kprateek.nayak@....com,
 wuyun.abel@...edance.com, youssefesmat@...omium.org, tglx@...utronix.de
Subject: Re: [PATCH 10/24] sched/uclamg: Handle delayed dequeue

On 9/11/24 10:10, Mike Galbraith wrote:
> On Wed, 2024-09-11 at 10:45 +0200, Peter Zijlstra wrote:
>> On Wed, Sep 11, 2024 at 09:35:16AM +0100, Luis Machado wrote:
>>>>
>>>> I'm assuming that removing the usage sites restores function?
>>>
>>> It does restore function if we remove the usage.
>>>
>>> From an initial look:
>>>
>>> cat /sys/kernel/debug/sched/debug | grep -i delay                                                                                                                                                                                                                             
>>>   .h_nr_delayed                  : -4
>>>   .h_nr_delayed                  : -6
>>>   .h_nr_delayed                  : -1
>>>   .h_nr_delayed                  : -6
>>>   .h_nr_delayed                  : -1
>>>   .h_nr_delayed                  : -1
>>>   .h_nr_delayed                  : -5
>>>   .h_nr_delayed                  : -6
>>>
>>> So probably an unexpected decrement or lack of an increment somewhere.
>>
>> Yeah, that's buggered. Ok, I'll go rebase sched/core and take this patch
>> out. I'll see if I can reproduce that.
> 
> Hm, would be interesting to know how the heck he's triggering that.
> 
> My x86_64 box refuses to produce any such artifacts with anything I've
> tossed at it, including full LTP with enterprise RT and !RT configs,
> both in master and my local SLE15-SP7 branch.  Hohum.
> 
> 	-Mike

>From what I can tell, the decrement that makes h_nr_delayed go negative is in
the dequeue_entities path.

First:

                if (!task_sleep && !task_delayed)
                        h_nr_delayed = !!se->sched_delayed;

h_nr_delayed is 1 here.

Then we decrement cfs_rq->h_nr_delayed below:

                cfs_rq->h_nr_running -= h_nr_running;
                cfs_rq->idle_h_nr_running -= idle_h_nr_running;
                cfs_rq->h_nr_delayed -= h_nr_delayed;