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Date:   Wed, 18 Apr 2018 12:54:28 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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>,
        Tejun Heo <htejun@gmail.com>, Christoph Lameter <cl@linux.com>
Subject: Re: [PATCH] Revert mm/vmstat.c: fix vmstat_update() preemption BUG
Message-Id: <20180418125428.206ae997096706eb9db1b7e2@linux-foundation.org>
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On Wed, 18 Apr 2018 17:44:36 +0200 Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> On 2018-04-11 21:07:29 [+0200], To Tejun Heo wrote:
> > On 2018-04-11 16:42:21 [+0200], To Tejun Heo wrote:
> > > > > 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.
> > 
> > This already happens:
> > - vmstat_shepherd() does get_online_cpus() and within this block it does
> >   queue_delayed_work_on(). So this has to wait until cpuhotplug
> >   completed before it can schedule something and then it won't schedule
> >   anything on the "off" CPU.
> > 
> > - The work item itself (vmstat_update()) schedules itself
> >   (conditionally) again.
> > 
> > - vmstat_cpu_down_prep() is the down event and does
> >   cancel_delayed_work_sync(). So it waits for the work-item to complete
> >   and cancels it.
> > 
> > This looks all good to me.
> > 
> > > > Thanks.

(top-posting repaired, Please don't do that - how am I supposed to
reply to you while maintaining appropriate context?)

> ping.
> any reason not to accept the revert?
> 

That will make the warnings come back.  Or was the hotplug issue
addressed by other means?  If so, that fix should be referred to in
the changelog.