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

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Date:	Wed, 10 Aug 2016 14:43:09 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	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>
Subject: Re: [PATCH] sched: fix the intention to re-evalute tick dependency
 for offline cpu

On Thu, Aug 04, 2016 at 05:51:20PM +0800, Wanpeng Li wrote:
> 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;
> +

I see, the CPU is offline but the tasks haven't been migrated yet.
That said it seems that rollback is still possible at this stage.

Somehow we may need to deal with it.

Thanks.

>  	/* Deadline tasks, even if single, need the tick */
>  	if (rq->dl.dl_nr_running)
>  		return false;
> -- 
> 1.9.1
>