Messages in this thread | | | Date | Tue, 30 Nov 2021 11:23:56 +0000 | From | Qais Yousef <> | Subject | Re: [PATCH] sched/uclamp: Fix rq->uclamp_max not set on first enqueue |
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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
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