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Date: Mon, 05 May 2014 20:06:06 +0200
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Frederic Weisbecker <fweisbec@gmail.com>
CC: linux-kernel@vger.kernel.org,
        Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>,
        Fernando Luis Vazquez Cao <fernando_b1@lab.ntt.co.jp>,
        Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH 4/4] nohz: Fix iowait overcounting if iowait task migrates
References: <1398365158-12568-1-git-send-email-dvlasenk@redhat.com> <1398365158-12568-4-git-send-email-dvlasenk@redhat.com> <20140429144727.GB6129@localhost.localdomain>
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On 04/29/2014 04:47 PM, Frederic Weisbecker wrote:
> On Thu, Apr 24, 2014 at 08:45:58PM +0200, Denys Vlasenko wrote:
>> +void tick_nohz_iowait_to_idle(int cpu)
>> +{
>> +	struct tick_sched *ts = tick_get_tick_sched(cpu);
>> +	ktime_t now = ktime_get();
>> +
>> +	write_seqcount_begin(&ts->idle_sleeptime_seq);
>> +	ts->iowait_exittime = now;
>> +	write_seqcount_end(&ts->idle_sleeptime_seq);
> 
> So now you have two concurrent updaters using the seqcount, which is
> very dangerous as the counters aren't updated atomically.
> 
> seqcount is only suitable when there is a single sequential updater.
> Once you deal with concurrent updaters you need seqlock.
> 
> And once you add seqlock in the hot scheduler path, you're hitting
> a big scalability issue.

What I need here is merely an atomic store.
The complication is, of course, that, ktime_t is not atomic[64]_t.

How do you think I can do an atomic store?