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 20:53:01 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Wanpeng Li <kernellwp@...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

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.