linux-kernel - Re: [accounting regression since rc1] scheduler updates

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Date:	Tue, 21 Aug 2007 11:11:16 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Jan Glauber <jang@...ux.vnet.ibm.com>,
	heiko.carstens@...ibm.com, Paul Mackerras <paulus@...ba.org>
Subject: Re: [accounting regression since rc1]  scheduler updates

On Tue, 2007-08-21 at 10:42 +0200, Ingo Molnar wrote:
> * Christian Borntraeger <borntraeger@...ibm.com> wrote:
> 
> > Am Montag, 20. August 2007 schrieb Ingo Molnar:
> > > could you send that precise sched_clock() patch? It should be an order 
> > > of magnitude simpler than the high-precision stime/utime tracking you 
> > > already do, and it's needed for quality scheduling anyway.
> > 
> > I have a question about that. I just played with sched_clock, and even 
> > when I intentionally slow down sched_clock by a factor of 2, my cpu 
> > bound process gets 100 % in top. If this is intentional, I dont 
> > understand how a virtualized sched_clock would fix the accounting 
> > change?
> 
> hm, does on s390 scheduler_tick() get driven in virtual time or in real 
> time? The very latest scheduler code will enforce a minimum rate of 
> sched_clock() across two scheduler_tick() calls (in rc3 and later 
> kernels). If sched_clock() "slows down" but scheduler_tick() still has a 
> real-time frequency then that impacts the quality of scheduling. So 
> scheduler_tick() and sched_clock() must really have the same behavior 
> (either both are virtual or both are real), so that scheduling becomes 
> invariant to steal-time.

scheduler_tick() is based on the HZ timer which uses the TOD clock =
real time. sched_clock() currently uses the TOD clock as well so in
regard to the new scheduler we currently do not have a problem. We have
a problem with cpu time accounting, the change to the /proc code breaks
the precise accounting on s390. To solve the cpu time accounting we need
to change sched_clock() to the cpu timer = virtual time. To change the
scheduler_tick() as well requires another patch and I fear it would
complicate things in the s390 backend.

And if you say that the scheduling becomes invariant to steal-time, how
is the cpu time accounting via sum_exec supposed to work if it does not
take steal-time into account ?

-- 
blue skies,
  Martin.

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


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