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

[Valentin Schneider]

On 30/11/21 11:23, Qais Yousef wrote:
> 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"?
>

Sounds good.

>>
>> > 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.
>

Oh, you're right, that initial uc_rq->value ends up being equivalent to
having the flag. Sorry for the confusion!

Patching up that original commit would only really be a "code correctness"
thing, it wouldn't fix any visible problem, so I think it's better to keep
your current Fixes:.

>>
>> > Signed-off-by: Qais Yousef <qais.yousef@xxxxxxx>
>>
>> Changelog nitpicking aside:
>> Reviewed-by: Valentin Schneider <Valentin.Schneider@xxxxxxx>
>
> Thanks!
>
> --
> Qais Yousef