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SubjectRe: [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy
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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.

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