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

[Dietmar Eggemann]

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()
> > > > ÂÂ -> sched_ttwu_pending()
> > > > ÂÂÂÂÂÂÂ -> ttwu_do_activate()ÂÂÂ => p->sched_remote_wakeup should be
> > > > false, so ENQUEUE_WAKEUP is set, ENQUEUE_MIGRATED is not
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂ -> ttwu_activate()
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ -> activate_task()
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ -> enqueue_task()
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ -> enqueue_task_fair()
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ -> enqueue_entity()
> > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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

[...]