linux-kernel - Re: [PATCH] Revert mm/vmstat.c: fix vmstat

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Date:   Wed, 27 Jun 2018 14:47:06 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Tejun Heo <htejun@...il.com>, Vlastimil Babka <vbabka@...e.cz>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, "Steven J . Hill" <steven.hill@...ium.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH] Revert mm/vmstat.c: fix vmstat_update() preemption BUG

On 2018-06-27 08:36:57 [-0400], Steven Rostedt wrote:
> On Wed, Apr 11, 2018 at 09:07:30PM +0200, Sebastian Andrzej Siewior wrote:
> > 
> > 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.
> 
> But can't we have something like this happen: ?
> 
> 	CPU0			CPU1			CPU2
> 	----			----			----
>  get_online_cpus()
>  queue_work(vmstat_update, cpu1)
>     wakeup(kworker/1)
> 			     High prio task running
>  put_online_cpus()
>  						     Shutdown CPU 1
> 			     migrate kworker/1
>  schedule kworker/1
>  (smp_processor_id() != 1)

I don't get CPU1 doing in here. kworker/1 should not be migrated to
begin with. The work item should be done before CPU2 shutdowns CPU1 (or
0) because vmstat_cpu_down_prep() cancels the work while the CPUs goes
down so it should not be migrated. Also, the work is enqueued while
holding the CPU HP lock.

Sebastian