linux-kernel - Re: [PATCH] sched/fair: vruntime should normalize when switching from fair

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Date:   Fri, 28 Sep 2018 19:35:53 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Steve Muckle <smuckle@...gle.com>, Wanpeng Li <kernellwp@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Miguel de Dios <migueldedios@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@...roid.com,
        Todd Kjos <tkjos@...gle.com>, Paul Turner <pjt@...gle.com>,
        quentin.perret@....com, Patrick Bellasi <Patrick.Bellasi@....com>,
        Chris.Redpath@....com, Morten Rasmussen <Morten.Rasmussen@....com>,
        John Dias <joaodias@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [PATCH] sched/fair: vruntime should normalize when switching from
 fair

On 09/28/2018 06:10 PM, Steve Muckle wrote:
> On 09/27/2018 05:43 PM, Wanpeng Li wrote:
>>>> On your CPU4:
>>>> scheduler_ipi()
>>>>    -> sched_ttwu_pending()
>>>>         -> ttwu_do_activate()    => p->sched_remote_wakeup should be
>>>> false, so ENQUEUE_WAKEUP is set, ENQUEUE_MIGRATED is not
>>>>              -> ttwu_activate()
>>>>                   -> activate_task()
>>>>                        -> enqueue_task()
>>>>                             -> enqueue_task_fair()
>>>>                                  -> enqueue_entity()
>>>>                                       bool renorm = !(flags &
>>>> ENQUEUE_WAKEUP) || (flags & ENQUEUE_MIGRATE)
>>>> so renorm is false in enqueue_entity(), why you mentioned that the
>>>> cfs_rq->min_vruntime is still added to the se->vruntime in
>>>> enqueue_task_fair()?
>>>
>>> Maybe this is a misunderstanding on my side but didn't you asked me to
>>> '... Could you point out when the fair rq's min_vruntime is added to the
>>> task's vruntime in your *later* scenario? ...'
>>
>> Yeah, if the calltrace above and my analysis is correct, then the fair
>> rq's min_vruntime will not be added to the task's vruntime in your
>> *later* scenario, which means that your patch is not necessary.
> 
> In the scenario I observed, the task is not waking - it is running and 
> being deboosted from priority inheritance, transitioning from RT to CFS.
> 
> Dietmar and I both were able to reproduce the issue with the testcase I 
> posted earlier in this thread.

Correct, and with the same testcase I got this call stack in this scenario:

[   35.588509] CPU: 1 PID: 2926 Comm: fair_task Not tainted 
4.18.0-rc6-00052-g11b7dafa2edb-dirty #5
[   35.597217] Hardware name: ARM Juno development board (r0) (DT)
[   35.603080] Call trace:
[   35.605509]  dump_backtrace+0x0/0x168
[   35.609138]  show_stack+0x24/0x30
[   35.612424]  dump_stack+0xac/0xe4
[   35.615710]  enqueue_task_fair+0xae0/0x11c0
[   35.619854]  rt_mutex_setprio+0x5a0/0x628
[   35.623827]  mark_wakeup_next_waiter+0x7c/0xc8
[   35.628228]  __rt_mutex_futex_unlock+0x30/0x50
[   35.632630]  do_futex+0x74c/0xb28
[   35.635912]  sys_futex+0x118/0x198
[   35.639280]  el0_svc_naked+0x30/0x34

[...]