Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1330558

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

Show all headers | View raw

[Multipart message — attachments visible in raw view] - view raw

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

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Re: [PATCH 4/4] sched,time: only call  account_{user,sys,guest,idle}_time once a jiffy Frederic Weisbecker <fweisbec@gmail.com> - 2016-02-09 18:20 +0100
  Re: [PATCH 4/4] sched,time: only call  account_{user,sys,guest,idle}_time once a jiffy Rik van Riel <riel@redhat.com> - 2016-02-09 19:20 +0100
  Re: [PATCH 4/4] sched,time: only call  account_{user,sys,guest,idle}_time once a jiffy Rik van Riel <riel@redhat.com> - 2016-02-09 22:30 +0100

csiph-web