Re: [PATCH 0/4] cputime: Virtual cputime accounting small cleanups and consolidation

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Thu, 21 Jun 2012 02:46:29 +0200
Frederic Weisbecker <fweisbec@gmail.com> wrote:

> 2012/6/21 Peter Zijlstra <peterz@infradead.org>:
> > On Tue, 2012-06-19 at 15:43 +0200, Frederic Weisbecker wrote:
> >>
> >> I wish we could do more vtime cputime accounting consolidation
> >> but archs do the things pretty differently although I bet the
> >> behaviour could be more unified.
> >>
> > Yes.. so s390,ia64 use thread_info, ppc uses their paca (arch private
> > precursor to per-cpu data).

s390 uses the prefix page / lowcore to accumulate some accounting information.
Which basically is per-cpu data with the advantage that it is accessible with
at address 0-8191 for each cpu. The entry code does not have to load a pointer
to get to that page, I would prefer NOT to use per-cpu data here.

> > So I understand why s390,ia64 want the sched hook, but I don't see why
> > ppc would need it, their account_process_tick() can fold whatever they
> > need on the tick.
> 
> I think in any case you need to flush the time on a descheduling task otherwise
> its pending time will be accounted later to the next task when it
> receives an irq.
> 
> So I fear we still need that sched switch hook even with per cpu data. This
> may be a simple account_system_vtime() call.

Indeed, the scheduler hook is needed to add the accumulated values to the
correct task. 
 
> > So I think reworking s390,ia64 to use per-cpu storage should get rid of
> > this switch hook altogether.

If we get rid of the switch hook then the quality of the accounting numbers
will get significantly worse. Think of the scenario of a long running
background process and a task that repeatedly wakes up for a very short
time. Without the scheduler hook chances are good that the background task
is accounted all the accumulated user time.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.