Re: Re: [PATCH 2/2] sched/fair: Fix premature check of WAKEUP_PREEMPTION

[Abel Wu]

Hi Madadi,

On 2/23/25 2:16 AM, Madadi Vineeth Reddy Wrote:
> On 21/02/25 21:27, Abel Wu wrote:
> > On 2/21/25 7:49 PM, Vincent Guittot Wrote:
> > > On Fri, 21 Feb 2025 at 12:12, Abel Wu <wuyun.abel@xxxxxxxxxxxxx> wrote:
> > > > Idle tasks are by definition preempted by non-idle tasks whether feat
> > > > WAKEUP_PREEMPTION is enabled or not. This isn't true any longer since
> > >
> > > I don't think it's true, only "sched_idle never preempts others" is
> > > always true but sched_feat(WAKEUP_PREEMPTION) is mainly there for
> > > debug purpose so if WAKEUP_PREEMPTION is false then nobody preempts
> > > others at wakeup, idle, batch or normal
> >
> > Hi Vincent, thanks for your comment!
> >
> > The SCHED_IDLE "definition" of being preempted by non-idle tasks comes
> > from commit 6bc912b71b6f ("sched: SCHED_OTHER vs SCHED_IDLE isolation")
> > which said:
> >
> >      - no SCHED_IDLE buddies
> >      - never let SCHED_IDLE preempt on wakeup
> >      - always preempt SCHED_IDLE on wakeup
> >      - limit SLEEPER fairness for SCHED_IDLE
> >
> > and that commit let it be preempted before checking WAKEUP_PREEMPTION.
> > The rules were introduced in 2009, and to the best of my knowledge there
> > seemed no behavior change ever since. Please correct me if I missed
> > anything.
>
> As Vincent mentioned, WAKEUP_PREEMPTION is primarily for debugging. Maybe
> it would help to document that SCHED_IDLE tasks are not preempted by non-idle
> tasks when WAKEUP_PREEMPTION is disabled. Otherwise, the intent of having no
> preemptions for debugging would be lost.
>
> Thoughts?

I am not sure I really understand the purpose of this debug feature.
If it wants to provide a way to check whether a performance degrade of
certain workload is due to overscheduling or not, then do we really
care about performance of SCHED_IDLE workloads and why?

IMHO preempting SCHED_IDLE before WAKEUP_PREEMPTION is to preserve the
IDLE semantics trying to behave like real idle task. It is somehow
weird to me that we treat sched-idle cpus as idle while don't let the
non-idle tasks run immediately on sched-idle cpus on debug case.

Thanks,
	Abel