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

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Date:	Fri, 12 Aug 2016 06:35:34 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Juri Lelli <juri.lelli@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"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>,
	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

2016-08-11 23:14 GMT+08:00 Juri Lelli <juri.lelli@....com>:
> On 11/08/16 16:45, Peter Zijlstra wrote:
>> 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
>>
>> Hurm, so this is after hot-unplug succeeded. We get a timer (which is
>> also already migrated), but we enqueue the dl task on the offline CPU,
>> because we need to do replenish because start_dl_timer() -- see the
>> comment in dl_task_timer() at #ifdef CONFIG_SMP.
>>
>> Then, once we've enqueued the task on the offline cpu, do we migrate it.
>>
>> Bit icky that, but I don't immediately see a better way.
>>
>
> [...]
>
>> Juri, any opinions?
>>
>
> So, we would need to do something like 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.
>
> dl_task_offline_migration() will still need to deactivate_task(), but
> that should be fine as we check RB_EMPTY_NODE() in __dequeue_dl_entity()
> and dequeue_pushable_dl_task().

Thanks, I will do it today. :)

Regards,
Wanpeng Li