linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Rik van Riel <riel@redhat.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: linux-kernel@vger.kernel.org, tglx@linutronix.de,
	mingo@kernel.org, luto@amacapital.net, peterz@infradead.org,
	clark@redhat.com, eric.dumazet@gmail.com
Subject: Re: [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy
Date: Tue, 09 Feb 2016 13:13:06 -0500	[thread overview]
Message-ID: <1455041586.15821.4.camel@redhat.com> (raw)
In-Reply-To: <20160209171139.GA7535@lerouge>

[-- Attachment #1: Type: text/plain, Size: 1599 bytes --]

On Tue, 2016-02-09 at 18:11 +0100, Frederic Weisbecker wrote:
> On Tue, Feb 02, 2016 at 12:19:46PM -0500, riel@redhat.com wrote:
> > From: Rik van Riel <riel@redhat.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

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 473 bytes --]

  reply	other threads:[~2016-02-09 18:13 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-02-02 17:19 [PATCH 0/4 v5] sched,time: reduce nohz_full syscall overhead 40% riel
2016-02-02 17:19 ` [PATCH 1/4] sched,time: remove non-power-of-two divides from __acct_update_integrals riel
2016-02-02 17:19 ` [PATCH 2/4] acct,time: change indentation in __acct_update_integrals riel
2016-02-02 17:19 ` [PATCH 3/4] time,acct: drop irq save & restore from __acct_update_integrals riel
2016-02-02 17:19 ` [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy riel
2016-02-09 17:11   ` Frederic Weisbecker
2016-02-09 18:13     ` Rik van Riel [this message]
2016-02-09 21:20     ` Rik van Riel
2016-02-08 21:07 ` [PATCH 0/4 v5] sched,time: reduce nohz_full syscall overhead 40% Rik van Riel
2016-02-09  2:13   ` Frederic Weisbecker
  -- strict thread matches above, loose matches on Subject: below --
2016-02-01 19:21 [PATCH 0/4 v4] " riel
2016-02-01 19:21 ` [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy riel
2016-02-01 20:00   ` Eric Dumazet
2016-02-01 20:08     ` Rik van Riel
2016-02-01  2:12 [PATCH 0/4 v3] sched,time: reduce nohz_full syscall overhead 40% riel
2016-02-01  2:12 ` [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy riel
2016-02-01  9:29   ` Peter Zijlstra
2016-02-01 19:23     ` Rik van Riel
2016-01-30  3:36 [PATCH 0/2] sched,time: reduce nohz_full syscall overhead 40% riel
2016-01-30  3:36 ` [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy riel

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=1455041586.15821.4.camel@redhat.com \
    --to=riel@redhat.com \
    --cc=clark@redhat.com \
    --cc=eric.dumazet@gmail.com \
    --cc=fweisbec@gmail.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=luto@amacapital.net \
    --cc=mingo@kernel.org \
    --cc=peterz@infradead.org \
    --cc=tglx@linutronix.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).