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    Subject[tip: sched/core] sched: Disable TTWU_QUEUE on RT
    The following commit has been merged into the sched/core branch of tip:

    Commit-ID: 539fbb5be0da56ffa1434b4f56521a0522bd1d61
    Gitweb: https://git.kernel.org/tip/539fbb5be0da56ffa1434b4f56521a0522bd1d61
    Author: Thomas Gleixner <tglx@linutronix.de>
    AuthorDate: Tue, 28 Sep 2021 14:24:27 +02:00
    Committer: Peter Zijlstra <peterz@infradead.org>
    CommitterDate: Tue, 05 Oct 2021 15:52:12 +02:00

    sched: Disable TTWU_QUEUE on RT

    The queued remote wakeup mechanism has turned out to be suboptimal for RT
    enabled kernels. The maximum latencies go up by a factor of > 5x in certain
    scenarious.

    This is caused by either long wake lists or by a large number of TTWU IPIs
    which are processed back to back.

    Disable it for RT.

    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Link: https://lkml.kernel.org/r/20210928122411.482262764@linutronix.de
    ---
    kernel/sched/features.h | 5 +++++
    1 file changed, 5 insertions(+)

    diff --git a/kernel/sched/features.h b/kernel/sched/features.h
    index 7f8dace..1cf435b 100644
    --- a/kernel/sched/features.h
    +++ b/kernel/sched/features.h
    @@ -46,11 +46,16 @@ SCHED_FEAT(DOUBLE_TICK, false)
    */
    SCHED_FEAT(NONTASK_CAPACITY, true)

    +#ifdef CONFIG_PREEMPT_RT
    +SCHED_FEAT(TTWU_QUEUE, false)
    +#else
    +
    /*
    * Queue remote wakeups on the target CPU and process them
    * using the scheduler IPI. Reduces rq->lock contention/bounces.
    */
    SCHED_FEAT(TTWU_QUEUE, true)
    +#endif

    /*
    * When doing wakeups, attempt to limit superfluous scans of the LLC domain.
    \
     
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