linux-kernel - Re: [PATCH v2] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count

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Date:	Thu, 30 Apr 2009 18:10:45 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, bharata@...ux.vnet.ibm.com,
	balajirrao@...il.com, dhaval@...ux.vnet.ibm.com,
	kamezawa.hiroyu@...fujitsu.com, a.p.zijlstra@...llo.nl,
	balbir@...ux.vnet.ibm.com, mingo@...e.hu, schwidefsky@...ibm.com,
	kosaki.motohiro@...fujitsu.com
Subject: Re: [PATCH v2] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu
 cputime count

On Thu, 30 Apr 2009 15:11:15 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> 
> Changelog:
>   since v1
>   - use percpu_counter_sum() instead percpu_counter_read()
> 
> 
> -------------------------------------
> Subject: [PATCH v2] cpuacct: VIRT_CPU_ACCOUNTING don't prevent percpu cputime count
> 
> cpuacct_update_stats() is called at every tick updating. and it use percpu_counter
> for avoiding performance degression.
> 
> For archs which define VIRT_CPU_ACCOUNTING, every tick would result
> in >1000 units of cputime updates and since this is much much greater
> than percpu_batch_counter, we end up taking spinlock on every tick.
> 
> This patch change batch rule. now, any cpu can store "percpu_counter_bach * jiffies"
> cputime in per-cpu cache.
> it mean this patch don't have behavior change if VIRT_CPU_ACCOUNTING=n.

Does this actually matter?

If we're calling cpuacct_update_stats() with large values of `cputime'
then presumably we're also calling cpuacct_update_stats() at a low
frequency, so the common lock-taking won't cause performance problems?

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