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 21:23:11 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <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

2016-08-10 20:43 GMT+08:00 Frederic Weisbecker <fweisbec@...il.com>:
> 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 for your review, Frederic. :) The rq lock is held to serialize
concurrent cpu hot-plug and dl task enqueue path(sched_can_stop_tick()
is called in this path), so I think there is no issue here.

Regards,
Wanpeng Li