linux-kernel - Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in cpuacct

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Date:	Wed, 18 Mar 2009 16:08:27 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	bharata@...ux.vnet.ibm.com
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, balbir@...ux.vnet.ibm.com,
	Li Zefan <lizf@...fujitsu.com>, linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in
 cpuacct_charge() safe when rcupreempt is used.

On Wed, 18 Mar 2009 10:18:01 +0530
Bharata B Rao <bharata@...ux.vnet.ibm.com> wrote:
> Looks ok to me, but will wait for experts' comments.
> 
> However, I did a quick measurement of read times with percpu_counter_read()
> (no readside lock) and percpu_counter_sum() (readside lock) and I don't
> see a major slowdown with percpu_counter_sum().
> 
> Time taken for 100 reads of cpuacct.stat with 1s delay b/n every read.
> percpu_counter_read() - 9845 us
> percpu_counter_sum() - 9974 us
> 
Then, almost 1 us overhead per read().....Hmm, seems big (as counter).

Thanks,
-Kame

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