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

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Message-ID: <20070821084243.GB1144@elte.hu>
Date:	Tue, 21 Aug 2007 10:42:43 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Jan Glauber <jang@...ux.vnet.ibm.com>,
	heiko.carstens@...ibm.com, Paul Mackerras <paulus@...ba.org>
Subject: Re: [accounting regression since rc1]  scheduler updates


* 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.

	Ingo
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