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

[Abel Wu <wuyun.abel@bytedance.com>]

On 2/21/25 7:49 PM, Vincent Guittot Wrote:
> On Fri, 21 Feb 2025 at 12:12, Abel Wu <wuyun.abel@bytedance.com> 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.

Best Regards,
	Abel