linux-kernel - Re: [PATCH] sched: fix the intention to re-evalute tick dependency for offline cpu

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Date:	Fri, 5 Aug 2016 13:36:33 +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] sched: fix the intention to re-evalute tick dependency
 for offline cpu

Cc Frederic,
2016-08-04 17:51 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.
>
> Actually we don't need to restart the tick since the target cpu is offline
> and nothing need scheduler tick. This patch fix it by not intend 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>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  kernel/sched/core.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7f2cae4..43b494f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -628,6 +628,9 @@ bool sched_can_stop_tick(struct rq *rq)
>  {
>         int fifo_nr_running;
>
> +       if (unlikely(!rq->online))
> +               return true;
> +
>         /* Deadline tasks, even if single, need the tick */
>         if (rq->dl.dl_nr_running)
>                 return false;
> --
> 1.9.1
>



-- 
Regards,
Wanpeng Li