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

[Wanpeng Li]

On Sat, 29 Sep 2018 at 01:36, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> wrote:
>
> 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

Thanks for pointing out. :)

Regards,
Wanpeng Li