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

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Message-ID: <20180627123657.2hb7ow4szjyhg5aj@home.goodmis.org>
Date:   Wed, 27 Jun 2018 08:36:57 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 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)

-- Steve