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Date: Wed, 07 May 2014 15:38:15 +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
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On 05/05/2014 08:06 PM, Denys Vlasenko wrote:
> 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?

Ok, I think a have a version which uses atomic64_t
and is not looking ugly. Sending new patchset now...