Re: [PATCH] sched/uclamp: Fix rq->uclamp_max not set on first enqueue

[Qais Yousef <qais.yousef@arm.com>]

Hi Valentin

On 11/26/21 10:51, Valentin Schneider wrote:
> On 25/11/21 16:52, Qais Yousef wrote:
> > Commit d81ae8aac85c ("sched/uclamp: Fix initialization of struct
> > uclamp_rq") introduced a bug where uclamp_max of the rq is not reset to
> > match the woken up task's uclamp_max when the rq is idle. This only
> > impacts the first wake up after enabling the static key. And it only
> 
> Wouldn't that rather be all wakeups after enabling the static key, until
> the rq goes idle and gains UCLAMP_FLAG_IDLE? e.g. if you enqueue N
> uclamp_max=512 tasks, the first enqueue flips the static key and the rq
> max-aggregate will stay at 1024 after the remaining enqueues.

Yep. Bad phrasing from my side. How about:

"This is visible from first wake up(s) until the first dequeue to idle after
enabling the static key"?

> 
> > matters if the uclamp_max of this task is < 1024 since only then its
> > uclamp_max will be effectively ignored.
> >
> > Fix it by properly initializing rq->uclamp_flags = UCLAMP_FLAG_IDLE to
> > ensure we reset rq uclamp_max when waking up from idle.
> >
> > Fixes: d81ae8aac85c ("sched/uclamp: Fix initialization of struct uclamp_rq")
> 
> Looking at this again, I'm starting to think this actually stems from the
> introduction of the flag:
> 
>   e496187da710 ("sched/uclamp: Enforce last task's UCLAMP_MAX")
> 
> Before the commit you point at, we would still initialize ->uclamp_flags to
> 0. This was probably hidden by all the activity at boot-time (e.g. just
> unparking smpboot threads) which yielded an nr_running>0 -> nr_running==0
> transition, IOW we'd most likely get UCLAMP_FLAG_IDLE set on a rq before
> any userspace task could get on there.
> 
> The static key would have only made this problem more visible.

Hmm. I can't see the sequence of events. I guess you could argue in theory that
this commit should have initialized the ->uclamp_flags to UCLAMP_FLAG_IDLE but
I think it used to work because uc_rq->value = 0 by default

	static inline void uclamp_rq_inc_id(struct rq *rq, struct task_struct *p,
					    enum uclamp_id clamp_id)
	{
		...

		if (uc_se->value > READ_ONCE(uc_rq->value))
			WRITE_ONCE(uc_rq->value, uc_se->value);
	}

The commit I point to changed makes uc_rq->value = 1024 by default, hence we
miss the first update.

I don't mind updating the FIXES tag here, though AFAICT there's no visible side
effect from it.

> 
> > Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> 
> Changelog nitpicking aside:
> Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com>

Thanks!

--
Qais Yousef