lkml.org 
[lkml]   [2022]   [Aug]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH 5.19 1025/1157] sched: Fix the check of nr_running at queue wakelist
    Date
    From: Tianchen Ding <dtcccc@linux.alibaba.com>

    [ Upstream commit 28156108fecb1f808b21d216e8ea8f0d205a530c ]

    The commit 2ebb17717550 ("sched/core: Offload wakee task activation if it
    the wakee is descheduling") checked rq->nr_running <= 1 to avoid task
    stacking when WF_ON_CPU.

    Per the ordering of writes to p->on_rq and p->on_cpu, observing p->on_cpu
    (WF_ON_CPU) in ttwu_queue_cond() implies !p->on_rq, IOW p has gone through
    the deactivate_task() in __schedule(), thus p has been accounted out of
    rq->nr_running. As such, the task being the only runnable task on the rq
    implies reading rq->nr_running == 0 at that point.

    The benchmark result is in [1].

    [1] https://lore.kernel.org/all/e34de686-4e85-bde1-9f3c-9bbc86b38627@linux.alibaba.com/

    Suggested-by: Valentin Schneider <vschneid@redhat.com>
    Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Valentin Schneider <vschneid@redhat.com>
    Link: https://lore.kernel.org/r/20220608233412.327341-2-dtcccc@linux.alibaba.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/core.c | 6 +++++-
    1 file changed, 5 insertions(+), 1 deletion(-)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index b4620354767b..6f1e3eef9346 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -3829,8 +3829,12 @@ static inline bool ttwu_queue_cond(int cpu, int wake_flags)
    * CPU then use the wakelist to offload the task activation to
    * the soon-to-be-idle CPU as the current CPU is likely busy.
    * nr_running is checked to avoid unnecessary task stacking.
    + *
    + * Note that we can only get here with (wakee) p->on_rq=0,
    + * p->on_cpu can be whatever, we've done the dequeue, so
    + * the wakee has been accounted out of ->nr_running.
    */
    - if ((wake_flags & WF_ON_CPU) && cpu_rq(cpu)->nr_running <= 1)
    + if ((wake_flags & WF_ON_CPU) && !cpu_rq(cpu)->nr_running)
    return true;

    return false;
    --
    2.35.1


    \
     
     \ /
      Last update: 2022-08-16 06:50    [W:7.233 / U:0.040 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site