linux-kernel - Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch values for stats counters

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Date:	Mon, 18 Jan 2010 20:55:27 +1100
From:	Anton Blanchard <anton@...ba.org>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>, mingo@...hat.com,
	hpa@...or.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl, balajirrao@...il.com,
	dhaval@...ux.vnet.ibm.com, tglx@...utronix.de,
	kamezawa.hiroyu@...fujitsu.com, akpm@...ux-foundation.org,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>, linux390@...ibm.com
Subject: Re: [PATCH] sched: cpuacct: Use bigger percpu counter batch values
 for stats counters


Hi Martin,

> For s390 the jiffies_to_cputime is a compile time constant. No need to
> initialize it at runtime, no? 

Indeed it is, I didn't look closely enough. Same with ia64 so no work to
do on either arch :)

> The patch itself trades some accuracy (larger cpu accounting value that
> are stored per-cpu) against runtime overhead (spinlock to transfer the
> value to the global variable in __percpu_counter_add). Did you
> calculate how big the loss in accuracy is?

The idea is we are already batching percpu_counter_batch jiffies, so
with CONFIG_VIRT_CPU_ACCOUNTING we batch the equivalent amount in
cputime.

Anton
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