lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  linux-cve-announce  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Date:	Thu, 24 Apr 2014 20:43:02 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	linux-kernel@...r.kernel.org
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 3/3] nohz: Fix iowait overcounting if iowait task migrates

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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Powered by blists - more mailing lists