linux-kernel - Re: [BUG nohz]: wrong user and system time accounting

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] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 30 Mar 2017 06:27:31 +0200
From:   Mike Galbraith <efault@....de>
To:     Rik van Riel <riel@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [BUG nohz]: wrong user and system time accounting

On Wed, 2017-03-29 at 16:08 -0400, Rik van Riel wrote:

> In other words, the tick on cpu0 is aligned
> with the tick on the nohz_full cpus, and
> jiffies is advanced while the nohz_full cpus
> with an active tick happen to be in kernel
> mode?

You really want skew_tick=1, especially on big boxen.
 
> Frederic, can you think of any reason why
> the tick on nohz_full CPUs would end up aligned
> with the tick on cpu0, instead of running at some
> random offset?

(I or low rq->clock bits as crude NOHZ collision avoidance)

> A random offset, or better yet a somewhat randomized
> tick length to make sure that simultaneous ticks are
> fairly rare and the vtime sampling does not end up
> "in phase" with the jiffies incrementing, could make
> the accounting work right again.

That improves jitter, especially on big boxen.  I have an 8 socket box
that thinks it's an extra large PC, there, collision avoidance matters
hugely.  I couldn't reproduce bean counting woes, no idea if collision
avoidance will help that.

	-Mike