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

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Date:	Tue, 14 Aug 2007 10:37:47 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
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

This is a 2nd try with correct email address, sorry for the duplicates.

Am Sonntag, 12. August 2007 schrieb Ingo Molnar:
> Linus, please pull the latest scheduler git tree from:

Hello Ingo,

this is a followup to the discussion in 
http://lkml.org/lkml/2007/7/19/538

Since 2.6.12, s390 already does precise accouting for system and user time.  
Depending on CONFIG_VIRT_CPU_ACCOUNTING, we  use two 64bit hardware timers on 
s390: the first returns the wall clock time and is stepped even if the 
virtual cpu is not backed by a physical cpu. The second timer is only 
stepped, when the virtual cpu is backed by a physical cpu. The timers have a 
very high accurancy, and the architecture guarantees that bit 51 is increased 
by one/microsecond. We store both timers on each context switch, irq, 
syscall, and machinecheck in entry.S. The calculation are made in 
arch/s390/kernel/vtime.c in accouting_system_vtime and friends with 
microsecond accurracy. This is also used for irq accouting (see the 
definition of irq_enter). It basically boils down to precise numbers in the 
cpu stat and the utime/stime for processes as well as knowledge about time 
stolen by the hypervisor. 

With CFS the accounting was changed, and everything is now based on 
sum_exec_runtime.  There is now an accounting regression on s390 (and maybe 
ppc64), as the default jiffy implemenation does not know anything about 
virtual cpus.

While looking for a solution, I started with a very quick hack and reverted 
b27f03d4bdc145a09fb7b0c0e004b29f1ee555fa for the procfs related changes. If I 
revert that commit, it seems that I get the old behaviour -  but of course 
this is just a hack.

I see some options now:

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. 
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). 
2. set sum_exec_runtime based on the precise utime and stime. Dont know enough 
about CFS if this would show different scheduling behaviour than 1
3. ifdef fs/proc/array.c depending on CONFIG_VIRT_CPU_ACCOUNTING. This will 
save some cycles, and the numbers are precise to a microsecond. Downside: the 
scheduler gets no information about virtual cpus and steal time so its 
probably not completely fair
4. implement sched_clock AND reuse the exisiting utime and stime numbers.
5. other clever solutions I cannot see

Any suggestions?

Christian
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