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>
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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
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next prev parent 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
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