Messages in this thread | | | From | Vincent Guittot <> | Date | Mon, 21 Jun 2021 10:38:36 +0200 | Subject | Re: [PATCH 1/2] sched/rt: Fix RT utilization tracking during policy change |
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On Fri, 18 Jun 2021 at 15:32, Vincent Donnefort <vincent.donnefort@arm.com> wrote: > > 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 will not see a such change, leaving the avg_rt structure outdated.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 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> > > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > index a525447..5ac0f31 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -2341,10 +2341,8 @@ 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 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) { > @@ -2355,6 +2353,13 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p) > if (p->prio < rq->curr->prio && cpu_online(cpu_of(rq))) > resched_curr(rq); > } > + > + /* > + * 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))
task_current(rq,p) equals the "rq->curr != p" above. Having 2 different ways to get the same value in the function is a bit confusing.
And call it once
Otherwise, the fix looks good to me
> + update_rt_rq_load_avg(rq_clock_pelt(rq), rq, 0); > } > > /* > -- > 2.7.4 >
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