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

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Message-ID: <20160811145733.GC4214@lerouge>
Date:	Thu, 11 Aug 2016 16:57:35 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...hat.com>,
	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 11, 2016 at 04:45:11PM +0200, 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.
> 
> And I think you're right in that we don't leak the nohz state, the
> migration, which we do immediately after this, takes care of that.

Are you sure there is no way the hotplug can fail after this stage?
I see this is at the end of the CPUHP_AP callbacks. Nothing else can
fail afterward?

If so then yes we are ok as migration takes care of the tick dependency.

Thanks.