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Message-ID: <20160811144511.GR6879@twins.programming.kicks-ass.net>
Date: Thu, 11 Aug 2016 16:45:11 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Wanpeng Li <kernellwp@...il.com>
Cc: 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>,
Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH] sched: fix the intention to re-evalute tick dependency
for offline cpu
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.
Juri, any opinions?
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