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    Subject[PATCH 5.13 340/800] sched/rt: Fix Deadline utilization tracking during policy change
    Date
    From: Vincent Donnefort <vincent.donnefort@arm.com>

    [ Upstream commit d7d607096ae6d378b4e92d49946d22739c047d4c ]

    DL keeps track of the utilization on a per-rq basis with the structure
    avg_dl. This utilization is updated during task_tick_dl(),
    put_prev_task_dl() and set_next_task_dl(). However, when the current
    running task changes its policy, set_next_task_dl() which would usually
    take care of updating the utilization when the rq starts running DL
    tasks, will not see a such change, leaving the avg_dl structure outdated.
    When that very same task will be dequeued later, put_prev_task_dl() will
    then update the utilization, based on a wrong last_update_time, leading to
    a huge spike in the DL utilization signal.

    The signal would eventually recover from this issue after few ms. Even
    if no DL tasks are run, avg_dl is also updated in
    __update_blocked_others(). But as the CPU capacity depends partly on the
    avg_dl, this issue has nonetheless a significant impact on the scheduler.

    Fix this issue by ensuring a load update when a running task changes
    its policy to DL.

    Fixes: 3727e0e ("sched/dl: Add dl_rq utilization tracking")
    Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
    Link: https://lore.kernel.org/r/1624271872-211872-3-git-send-email-vincent.donnefort@arm.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/deadline.c | 2 ++
    1 file changed, 2 insertions(+)

    diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
    index 9a2989749b8d..2f9964b467e0 100644
    --- a/kernel/sched/deadline.c
    +++ b/kernel/sched/deadline.c
    @@ -2486,6 +2486,8 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
    check_preempt_curr_dl(rq, p, 0);
    else
    resched_curr(rq);
    + } else {
    + update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 0);
    }
    }

    --
    2.30.2


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