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    SubjectRe: [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@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
    >

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