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

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Date:	Mon, 20 Aug 2007 17:45:29 +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:

> 1. Jan could finish his sched_clock implementation for s390 and we 
> would get close to the precise numbers. This would also let CFS make 
> better decisions. [...]

i think this is the best option and it should give us the same /proc 
accuracy on s390 as before, plus improved scheduler precision. (and 
improved tracing accuracy, etc. etc.) Note that for architectures that 
already have sched_clock() at least as precise as the stime/utime stats 
there's no problem - and that seems to include all architectures except 
s390.

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.

> [...] Downside: its not as precise as before as we do some math on the 
> numbers and it will burn cycles to compute numbers we already have 
> (utime=sum*utime/stime).

i can see no real downside to it: if all of stime, utime and 
sum_exec_clock are precise, then the numbers we present via /proc are 
precise too:

   sum_exec * utime / stime;

there should be no loss of precision on s390 because the 
multiplication/division rounding is not accumulating - we keep the 
precise sum_exec, utime and stime values untouched.

on x86 we dont really want to slow down every irq and syscall event with 
precise stime/utime stats for 'top' to display. On s390 the 
multiplication and division is indeed superfluous but it keeps the code 
generic for arches where utime/stime is less precise and irq-sampled - 
while the sum is always precise. It also animates architectures that 
have an imprecise sched_clock() implementation to improve its accuracy. 
Accessing the /proc files alone is many orders of magnitude more 
expensive than this simple multiplication and division.

	Ingo

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