linux-kernel - Re: [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}

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]

Message-ID: <1455041586.15821.4.camel@redhat.com>
Date:	Tue, 09 Feb 2016 13:13:06 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
	luto@...capital.net, peterz@...radead.org, clark@...hat.com,
	eric.dumazet@...il.com
Subject: Re: [PATCH 4/4] sched,time: only call
 account_{user,sys,guest,idle}_time once a jiffy

On Tue, 2016-02-09 at 18:11 +0100, Frederic Weisbecker wrote:
> On Tue, Feb 02, 2016 at 12:19:46PM -0500, riel@...hat.com wrote:
> > From: Rik van Riel <riel@...hat.com>
> > 
> > After removing __acct_update_integrals from the profile,
> > native_sched_clock remains as the top CPU user. This can be
> > reduced by only calling account_{user,sys,guest,idle}_time
> > once per jiffy for long running tasks on nohz_full CPUs.
> > 
> > This will reduce timing accuracy on nohz_full CPUs to jiffy
> > based sampling, just like on normal CPUs.
> 
> Now after this patch, the scenario is rather different. We are
> accounting the
> real time spent in a slice with a similar probablity.
> This becomes: T_slice * P(T_slice*HZ).
> 
> So it seems it could result into logarithmic accounting: timeslices
> of 1 second
> will be accounted right whereas repeating tiny timeslices may result
> in much lower
> values than expected.

You are right that this code does not handle
short timeslices well.

However, I believe it does not have to, because
the scheduler already takes care of that.

At context switch time, the scheduler will call
vtime_common_task_switch, which calls
arch_vtime_task_switch, which handles precise
time accounting at task switch time.

The call chain is like this:

arch_vtime_task_switch
vtime_common_task_switch
vtime_task_switch
finish_task_switch
context_switch
__schedule

As you can see, the time accounting for shorter
running tasks is already handled by other code,
which means my patch should be ok.

-- 
All rights reversed

Download attachment "signature.asc" of type "application/pgp-signature" (474 bytes)