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

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Message-ID: <CANRm+CwoLZpty7dfh7pdW8PVY9JHq0hWm1AR2GtTXDHNeb=uzA@mail.gmail.com>
Date:	Thu, 11 Aug 2016 09:36:19 +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

2016-08-11 2:53 GMT+08:00 Frederic Weisbecker <fweisbec@...il.com>:
> On Wed, Aug 10, 2016 at 09:23:11PM +0800, Wanpeng Li wrote:
>> 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.
>
> It's not about concurrency though. It's rather that if the CPU runs
> tickless, does cpu_down() and fails, then if the dl task needs the tick and
> we ignore the IPI due to cpu_is_offline(), we may be still running tickless
> forever after cpu_down() failure exit.

If the cpu is offilne when the dl task timer fires, dl task will be
migrated to another suitable cpu, so there is no issue if cpu
hot-unplug fail and online again.

Regards,
Wanpeng Li