linux-kernel - Re: [PATCH v3] sched/deadline: Fix the intention to re-evalute tick dependency for offline cpu

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Date:	Tue, 16 Aug 2016 10:04:38 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@....com>,
	Luca Abeni <luca.abeni@...tn.it>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v3] sched/deadline: Fix the intention to re-evalute tick
 dependency for offline cpu

Ping Juri, Frederic, could I get your Acked?
2016-08-12 17:24 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> From: Wanpeng Li <wanpeng.li@...mail.com>
>
> The dl task will be replenished after dl task timer fire and start a
> new period. It will be enqueued and to re-evaluate its dependency on
> the tick in order to restart it. However, if cpu is hot-unplug,
> irq_work_queue will splash since the target cpu is offline.
>
> As a result:
>
>     WARNING: CPU: 2 PID: 0 at kernel/irq_work.c:69 irq_work_queue_on+0xad/0xe0
>     Call Trace:
>      dump_stack+0x99/0xd0
>      __warn+0xd1/0xf0
>      warn_slowpath_null+0x1d/0x20
>      irq_work_queue_on+0xad/0xe0
>      tick_nohz_full_kick_cpu+0x44/0x50
>      tick_nohz_dep_set_cpu+0x74/0xb0
>      enqueue_task_dl+0x226/0x480
>      activate_task+0x5c/0xa0
>      dl_task_timer+0x19b/0x2c0
>      ? push_dl_task.part.31+0x190/0x190
>
> This can be triggered by hot-unplug the full dynticks cpu which dl
> task is running on.
>
> We enqueue the dl task on the offline CPU, because we need to do
> replenish for start_dl_timer(). So, as Juri pointed out, we would
> need to do is calling replenish_dl_entity() directly, instead of
> enqueue_task_dl(). pi_se shouldn't be a problem as the task shouldn't
> be boosted if it was throttled.
>
> This patch fix it by just replenish dl entity to avoid the intention
> to re-evaluate tick dependency if the cpu is offline.
>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@....com>
> Cc: Luca Abeni <luca.abeni@...tn.it>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
> v2 -> v3:
>  * move rq->online check under CONFIG_SMP
> v1 -> v2:
>  * replenish dl entity
>
>  kernel/sched/deadline.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index d091f4a..ce0fb00 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -641,6 +641,11 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>                 goto unlock;
>         }
>
> +#ifdef CONFIG_SMP
> +       if (unlikely(!rq->online))
> +               goto offline;
> +#endif
> +
>         enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
>         if (dl_task(rq->curr))
>                 check_preempt_curr_dl(rq, p, 0);
> @@ -648,6 +653,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>                 resched_curr(rq);
>
>  #ifdef CONFIG_SMP
> +offline:
>         /*
>          * Perform balancing operations here; after the replenishments.  We
>          * cannot drop rq->lock before this, otherwise the assertion in
> @@ -659,6 +665,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>          * XXX figure out if select_task_rq_dl() deals with offline cpus.
>          */
>         if (unlikely(!rq->online)) {
> +               replenish_dl_entity(dl_se, dl_se);
>                 lockdep_unpin_lock(&rq->lock, rf.cookie);
>                 rq = dl_task_offline_migration(rq, p);
>                 rf.cookie = lockdep_pin_lock(&rq->lock);
> --
> 1.9.1
>



-- 
Regards,
Wanpeng Li