Re: [PATCH RESEND v10] sched/deadline: support dl task migration during cpu hotplug

[Ingo Molnar <mingo@kernel.org>]

* Wanpeng Li <wanpeng.li@linux.intel.com> 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 <juri.lelli@arm.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
> ---
>  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