From: Denys Vlasenko <dvlasenk@redhat.com>
To: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>,
linux-kernel@vger.kernel.org
Cc: Frederic Weisbecker <fweisbec@gmail.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 3/3] nohz: Fix iowait overcounting if iowait task migrates
Date: Thu, 24 Apr 2014 20:43:02 +0200 [thread overview]
Message-ID: <53595B36.70606@redhat.com> (raw)
In-Reply-To: <5358C909.6010208@jp.fujitsu.com>
On 04/24/2014 10:19 AM, Hidetoshi Seto wrote:
> As I already mentioned in previous discussion with Peter, I have
> concern here that this change might have impact on performance.
> Especially in case if system is a kind of io-busy box, originally
> there may be no iowait time (and possibly also no idle time).
> For such case this change adds extra execution cost to manage
> value of iowait_exittime which might not used.
Everything has some cost. Correctness usually trumps a few extra
locked bus cycles.
Wouldn't it be nice if we'd know whether anyone even needs
the stats? If no one going to read /proc/stat on the box,
there's no point in going to all the trouble to maintain the counters...
> And if we successfully found a way to get the iowait_exittime
> within reasonable negligible cheap cost, then why we don't use
> it for NOHZ=n kernels too?
Kernels without NOHZ maintain the counters based on timer interrupt
sampling. It should still work fine.
> As Frederic already pointed, seqcount must be better choice.
Yes, I'm switching to seqcounts.
prev parent reply other threads:[~2014-04-24 18:43 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-04-23 19:00 [PATCH 1/3] nohz: Only update sleeptime stats locally Denys Vlasenko
2014-04-23 19:00 ` [PATCH 2/3] nohz: Synchronize sleep time stats with memory barriers Denys Vlasenko
2014-04-24 0:16 ` Frederic Weisbecker
2014-04-24 18:36 ` Denys Vlasenko
2014-04-23 19:00 ` [PATCH 3/3] nohz: Fix iowait overcounting if iowait task migrates Denys Vlasenko
2014-04-24 0:37 ` Frederic Weisbecker
2014-04-24 7:07 ` Peter Zijlstra
2014-04-24 18:38 ` Denys Vlasenko
2014-04-24 8:19 ` Hidetoshi Seto
2014-04-24 18:43 ` Denys Vlasenko [this message]
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