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    Subject[PATCH 5.13 339/800] sched/rt: Fix RT utilization tracking during policy change
    Date
    From: Vincent Donnefort <vincent.donnefort@arm.com>

    [ Upstream commit fecfcbc288e9f4923f40fd23ca78a6acdc7fdf6c ]

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    will not see a such change, leaving the avg_rt structure outdated.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

    The signal would eventually recover from this issue after few ms. Even if
    no RT tasks are run, avg_rt is also updated in __update_blocked_others().
    But as the CPU capacity depends partly on the avg_rt, 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 RT.

    Fixes: 371bf427 ("sched/rt: Add rt_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/1624271872-211872-2-git-send-email-vincent.donnefort@arm.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/rt.c | 17 ++++++++++++-----
    1 file changed, 12 insertions(+), 5 deletions(-)

    diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
    index c286e5ba3c94..3b1b8b025b74 100644
    --- a/kernel/sched/rt.c
    +++ b/kernel/sched/rt.c
    @@ -2331,13 +2331,20 @@ void __init init_sched_rt_class(void)
    static void switched_to_rt(struct rq *rq, struct task_struct *p)
    {
    /*
    - * If we are already running, then there's nothing
    - * that needs to be done. But if we are not running
    - * we may need to preempt the current running task.
    - * If that current running task is also an RT task
    + * If we are running, update the avg_rt tracking, as the running time
    + * will now on be accounted into the latter.
    + */
    + if (task_current(rq, p)) {
    + update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 0);
    + return;
    + }
    +
    + /*
    + * If we are not running we may need to preempt the current
    + * running task. If that current running task is also an RT task
    * then see if we can move to another run queue.
    */
    - if (task_on_rq_queued(p) && rq->curr != p) {
    + if (task_on_rq_queued(p)) {
    #ifdef CONFIG_SMP
    if (p->nr_cpus_allowed > 1 && rq->rt.overloaded)
    rt_queue_push_tasks(rq);
    --
    2.30.2


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