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Date: Thu, 10 Jun 2021 21:05:12 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Quentin Perret <qperret@...gle.com> Cc: mingo@...hat.com, vincent.guittot@...aro.org, dietmar.eggemann@....com, qais.yousef@....com, rickyiu@...gle.com, wvw@...gle.com, patrick.bellasi@...bug.net, xuewen.yan94@...il.com, linux-kernel@...r.kernel.org, kernel-team@...roid.com Subject: Re: [PATCH v2 1/3] sched: Fix UCLAMP_FLAG_IDLE setting On Thu, Jun 10, 2021 at 03:13:04PM +0000, Quentin Perret wrote: > The UCLAMP_FLAG_IDLE flag is set on a runqueue when dequeueing the last > active task to maintain the last uclamp.max and prevent blocked util > from suddenly becoming visible. > > However, there is an asymmetry in how the flag is set and cleared which > can lead to having the flag set whilst there are active tasks on the rq. > Specifically, the flag is cleared in the uclamp_rq_inc() path, which is > called at enqueue time, but set in uclamp_rq_dec_id() which is called > both when dequeueing a task _and_ in the update_uclamp_active() path. As > a result, when both uclamp_rq_{dec,ind}_id() are called from > update_uclamp_active(), the flag ends up being set but not cleared, > hence leaving the runqueue in a broken state. > > Fix this by setting the flag in the uclamp_rq_inc_id() path to ensure > things remain symmetrical. The code you moved is neither in uclamp_rq_inc_id(), although uclamp_idle_reset() is called from there, nor does it _set_ the flag. I'm thinking it's been a long warm day? ;-) > > Fixes: e496187da710 ("sched/uclamp: Enforce last task's UCLAMP_MAX") > Reported-by: Rick Yiu <rickyiu@...gle.com> > Reviewed-by: Qais Yousef <qais.yousef@....com> > Signed-off-by: Quentin Perret <qperret@...gle.com> > --- > kernel/sched/core.c | 5 +---- > 1 file changed, 1 insertion(+), 4 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 5226cc26a095..3b213402798e 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -980,6 +980,7 @@ static inline void uclamp_idle_reset(struct rq *rq, enum uclamp_id clamp_id, > if (!(rq->uclamp_flags & UCLAMP_FLAG_IDLE)) > return; > > + rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE; > WRITE_ONCE(rq->uclamp[clamp_id].value, clamp_value); > } > > @@ -1252,10 +1253,6 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p) > > for_each_clamp_id(clamp_id) > uclamp_rq_inc_id(rq, p, clamp_id); > - > - /* Reset clamp idle holding when there is one RUNNABLE task */ > - if (rq->uclamp_flags & UCLAMP_FLAG_IDLE) > - rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE; > } > > static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p) > -- > 2.32.0.272.g935e593368-goog >
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