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: Fri, 28 Sep 2018 19:11:50 +0200	[thread overview]
Message-ID: <e055dace-be32-e074-ab35-dfa84534d466@arm.com> (raw)
In-Reply-To: <CANRm+CyJQ_D9BP70OmB6z6r0zxDPoB0y828+E0tqis0x=jsd5A@mail.gmail.com>

On 09/28/2018 02:43 AM, Wanpeng Li wrote:
> On Thu, 27 Sep 2018 at 21:23, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>>
>> 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? ...'
> 
> 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.

Ah, ok, both, the ENQUEUE_WAKEUP and the ENQUEUE_MIGRATED are not set in 
the enqueue_entity() call so renorm is 1 (flags is 0xe).