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From: Tejun Heo <htejun@gmail.com>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	tglx@linutronix.de, "Steven J . Hill" <steven.hill@cavium.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Lameter <cl@linux.com>
Subject: Re: [PATCH] Revert mm/vmstat.c: fix vmstat_update() preemption BUG
Date: Wed, 11 Apr 2018 07:09:13 -0700	[thread overview]
Message-ID: <20180411140913.GE793541@devbig577.frc2.facebook.com> (raw)
In-Reply-To: <ef663b6d-9e9f-65c6-25ec-ffa88347c58d@suse.cz>

Hello,

On Wed, Apr 11, 2018 at 03:56:43PM +0200, Vlastimil Babka wrote:
> > vmstat_update() is invoked by a kworker on a specific CPU. This worker
> > it bound to this CPU. The name of the worker was "kworker/1:1" so it
> > should have been a worker which was bound to CPU1. A worker which can
> > run on any CPU would have a `u' before the first digit.
> 
> Oh my, and I have just been assured by Tejun that his cannot happen :)
> And yet, in the original report [1] I see:
> 
> CPU: 0 PID: 269 Comm: kworker/1:1 Not tainted
> 
> So is this perhaps related to the cpu hotplug that [1] mentions? e.g. is
> the cpu being hotplugged cpu 1, the worker started too early before
> stuff can be scheduled on the CPU, so it has to run on different than
> designated CPU?
> 
> [1] https://marc.info/?l=linux-mm&m=152088260625433&w=2

The report says that it happens when hotplug is attempted.  Per-cpu
doesn't pin the cpu alive, so if the cpu goes down while a work item
is in flight or a work item is queued while a cpu is offline it'll end
up executing on some other cpu.  So, if a piece of code doesn't want
that happening, it gotta interlock itself - ie. start queueing when
the cpu comes online and flush and prevent further queueing when its
cpu goes down.

Thanks.

-- 
tejun

  reply	other threads:[~2018-04-11 14:09 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-11  9:57 [PATCH] Revert mm/vmstat.c: fix vmstat_update() preemption BUG Sebastian Andrzej Siewior
2018-04-11 13:56 ` Vlastimil Babka
2018-04-11 14:09   ` Tejun Heo [this message]
2018-04-11 14:42     ` Sebastian Andrzej Siewior
2018-04-11 19:07       ` Sebastian Andrzej Siewior
2018-04-18 15:44         ` Sebastian Andrzej Siewior
2018-04-18 19:54           ` Andrew Morton
2018-06-27 12:36         ` Steven Rostedt
2018-06-27 12:47           ` Sebastian Andrzej Siewior

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