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

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 30 Apr 2009 15:11:15 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@jp.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?