From: Peter Zijlstra <peterz@infradead.org>
To: Quentin Perret <qperret@google.com>
Cc: mingo@redhat.com, vincent.guittot@linaro.org,
dietmar.eggemann@arm.com, qais.yousef@arm.com,
rickyiu@google.com, wvw@google.com, patrick.bellasi@matbug.net,
xuewen.yan94@gmail.com, linux-kernel@vger.kernel.org,
kernel-team@android.com
Subject: Re: [PATCH v2 1/3] sched: Fix UCLAMP_FLAG_IDLE setting
Date: Thu, 10 Jun 2021 21:05:12 +0200 [thread overview]
Message-ID: <YMJiaO0IN2pN/EYY@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210610151306.1789549-2-qperret@google.com>
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@google.com>
> Reviewed-by: Qais Yousef <qais.yousef@arm.com>
> Signed-off-by: Quentin Perret <qperret@google.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
>
next prev parent reply other threads:[~2021-06-10 19:05 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-10 15:13 [PATCH v2 0/3] A few uclamp fixes Quentin Perret
2021-06-10 15:13 ` [PATCH v2 1/3] sched: Fix UCLAMP_FLAG_IDLE setting Quentin Perret
2021-06-10 19:05 ` Peter Zijlstra [this message]
2021-06-11 7:25 ` Quentin Perret
2021-06-17 15:27 ` Dietmar Eggemann
2021-06-21 10:57 ` Quentin Perret
2021-06-10 15:13 ` [PATCH v2 2/3] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS Quentin Perret
2021-06-10 19:15 ` Peter Zijlstra
2021-06-11 8:59 ` Quentin Perret
2021-06-11 9:07 ` Quentin Perret
2021-06-11 9:20 ` Peter Zijlstra
2021-06-10 15:13 ` [PATCH v2 3/3] sched: Make uclamp changes depend on CAP_SYS_NICE Quentin Perret
2021-06-11 12:48 ` Qais Yousef
2021-06-11 13:08 ` Quentin Perret
2021-06-11 13:26 ` Qais Yousef
2021-06-11 13:49 ` Quentin Perret
2021-06-11 14:17 ` Qais Yousef
2021-06-11 14:43 ` Quentin Perret
2021-06-14 15:03 ` Qais Yousef
2021-06-21 10:52 ` Quentin Perret
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