Re: Re: [PATCH v2 2/2] sched/fair: Fix premature check of WAKEUP_PREEMPTION

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

On 2/28/25 10:26 PM, Vincent Guittot wrote:
> On Thu, 27 Feb 2025 at 09:58, Abel Wu <wuyun.abel@bytedance.com> wrote:
>>
>> The commit 6bc912b71b6f ("sched: SCHED_OTHER vs SCHED_IDLE isolation")
>> defines the behavior of SCHED_IDLE as following:
>>
>>   - no SCHED_IDLE buddies
>>   - never let SCHED_IDLE preempt on wakeup
>>   - always preempt SCHED_IDLE on wakeup
>>   - limit SLEEPER fairness for SCHED_IDLE
>>
>> and the 3rd rule is broken if !WAKEUP_PREEMPTION due to recently merged
>> commit faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")
>>
>> Although WAKEUP_PREEMPTION is mainly there for debug purpose to provide
>> a way to check whether a performance degrade of certain workload is due
>> to overscheduling or not, it is still kind of weird that we treat sched-
>> idle cpus as idle but don't let the non-idle tasks preempt the sched-idle
>> cpus in debug mode (!WAKEUP_PREEMPTION).
>>
>> Fix it by strictly following the aforementioned rules.
>>
>> Fixes: faa42d29419d ("sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy")
>> Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
> 
> WAKEUP_PREEMPTION seems to be still used so
> 
> Acked-by: Vincent Guittot <vincent.guittot@linaro.org>

Thanks!