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Date:   Wed, 11 Apr 2018 16:42:21 +0200
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Tejun Heo <htejun@gmail.com>
Cc:     Vlastimil Babka <vbabka@suse.cz>, 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
Message-ID: <20180411144221.o3v73v536tpnc6n3@linutronix.de>
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On 2018-04-11 07:09:13 [-0700], Tejun Heo wrote:
> 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.

I missed that cpuhotplug part while reading it. So in that case, let me
add a CPU-hotplug notifier which cancels that work. After all it is not
need once the CPU is gone.

> Thanks.
> 

Sebastian