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

[Dietmar Eggemann]

On 09/28/2018 02:43 AM, Wanpeng Li wrote:
> On Thu, 27 Sep 2018 at 21:23, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
> > On 09/27/2018 03:19 AM, Wanpeng Li wrote:
> > > On Thu, 27 Sep 2018 at 06:38, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> 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@xxxxxxx> 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).