* Wanpeng Li <[email protected]> wrote:
> I observe that dl task can't be migrated to other cpus during cpu hotplug,
> in addition, task may/may not be running again if cpu is added back. The
> root cause which I found is that dl task will be throtted and removed from
> dl rq after comsuming all budget, which leads to stop task can't pick it up
> from dl rq and migrate to other cpus during hotplug.
>
> The method to reproduce:
> schedtool -E -t 50000:100000 -e ./test
> Actually test is just a simple for loop. Then observe which cpu the test
> task is on.
> echo 0 > /sys/devices/system/cpu/cpuN/online
>
> This patch adds the dl task migration during cpu hotplug by finding a most
> suitable later deadline rq after dl timer fire if current rq is offline,
> if fail to find a suitable later deadline rq then fallback to any eligible
> online cpu in order that the deadline task will come back to us, and the
> push/pull mechanism should then move it around properly.
>
> Suggested-and-acked-by: Juri Lelli <[email protected]>
> Signed-off-by: Wanpeng Li <[email protected]>
> ---
> kernel/sched/deadline.c | 54
> +++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 54 insertions(+)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 5cb5c9c..db457b9 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -492,6 +492,7 @@ static int start_dl_timer(struct sched_dl_entity *dl_se,
> bool boosted)
> return hrtimer_active(&dl_se->dl_timer);
> }
>
> +static struct rq *find_lock_later_rq(struct task_struct *task, struct rq
> *rq);
> /*
> * This is the bandwidth enforcement timer callback. If here, we know
> * a task is not on its dl_rq, since the fact that the timer was running
> @@ -537,6 +538,59 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer
> *timer)
> update_rq_clock(rq);
>
> /*
> + * So if we find that the rq the task was on is no longer
> + * available, we need to select a new rq.
> + */
> + if (unlikely(!rq->online)) {
> + struct rq *later_rq = NULL;
> + bool fallback = false;
> +
> + later_rq = find_lock_later_rq(p, rq);
> +
> + if (!later_rq) {
> + int cpu;
> +
> + /*
> + * If cannot preempt any rq, fallback to pick any
> + * online cpu.
s/If cannot/If we cannot
s/fallback/fall back
> + */
> + fallback = true;
> + cpu = cpumask_any_and(cpu_active_mask,
> + tsk_cpus_allowed(p));
shouldn't be on separate lines - but this is also a sign that the guts
of this new code should be in a helper function, not inside several
layers of branches.
> + if (cpu >= nr_cpu_ids) {
> + if (dl_bandwidth_enabled()) {
> + /*
> + * Fail to find any suitable cpu.
> + * The task will never come back!
> + */
> + WARN_ON(1);
Can this condition happen to users with a non-buggy kernel?
Thanks,
Ingo
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