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

From: Dietmar Eggemann <dietmar.eggemann@arm.com>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Steve Muckle <smuckle@google.com>,
	Miguel de Dios <migueldedios@google.com>,
	Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel-team@android.com, Todd Kjos <tkjos@google.com>,
	Paul Turner <pjt@google.com>,
	quentin.perret@arm.com, Patrick Bellasi <Patrick.Bellasi@arm.com>,
	Chris.Redpath@arm.com,
	Morten Rasmussen <Morten.Rasmussen@arm.com>,
	John Dias <joaodias@google.com>,
	Wanpeng Li <wanpengli@tencent.com>
Subject: Re: [PATCH] sched/fair: vruntime should normalize when switching from fair
Date: Thu, 27 Sep 2018 15:22:47 +0200	[thread overview]
Message-ID: <fab279dd-0f6c-af7a-5a06-d26e77afad6a@arm.com> (raw)
In-Reply-To: <CANRm+CzasXnjUtA_Xkm27=YDArc-gcpLmpSHv1cAqxihfocxRw@mail.gmail.com>

On 09/27/2018 03:19 AM, Wanpeng Li wrote:
> On Thu, 27 Sep 2018 at 06:38, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>>
>> Hi,
>>
>> On 09/26/2018 11:50 AM, Wanpeng Li wrote:
>>> Hi Dietmar,
>>> On Tue, 28 Aug 2018 at 22:55, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>>>>
>>>> On 08/27/2018 12:14 PM, Peter Zijlstra wrote:
>>>>> On Fri, Aug 24, 2018 at 02:24:48PM -0700, Steve Muckle wrote:
>>>>>> On 08/24/2018 02:47 AM, Peter Zijlstra wrote:
>>>>>>>>> On 08/17/2018 11:27 AM, Steve Muckle wrote:
>>
>> [...]
>>
>>>>>>>>>> - later, when the prio is deboosted and the task is moved back
>>>>>>>>>>        to the fair class, the fair rq's min_vruntime is added to
>>>>>>>>>>        the task's vruntime, even though it wasn't subtracted earlier.
>>>
>>> Could you point out when the fair rq's min_vruntime is added to the
>>> task's vruntime in your *later* scenario? attach_task_cfs_rq will not
>>> do that the same reason as detach_task_cfs_rq. fair task's
>>> sched_remote_wakeup is false which results in vruntime will not be
>>> renormalized in enqueue_entity.
>>
>> The cfs_rq->min_vruntime is still added to the se->vruntime in
>> enqueue_task_fair().
> 
> I understand what your patch done,

It's not my patch ;-) I just helped to find out under which 
circumstances this issue can happen.

> 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? ...'