linux-kernel - Re: [PATCH] remove rq->lock from cpuacct cgroup v2

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Message-ID: <344eb09a0903040411n12379babjb62d31d28b77d1e6@mail.gmail.com>
Date:	Wed, 4 Mar 2009 17:41:13 +0530
From:	Bharata B Rao <bharata.rao@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	paulmck@...ux.vnet.ibm.com, Li Zefan <lizf@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Menage <menage@...gle.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>, kenchen@...gle.com
Subject: Re: [PATCH] remove rq->lock from cpuacct cgroup v2

On Wed, Mar 4, 2009 at 1:50 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Wed, 4 Mar 2009 13:24:43 +0530
> Bharata B Rao <bharata.rao@...il.com> wrote:
>
>> Instead of subsystems handling all these percpu counter problems
>> themselves, shouldn't we be using percpu_counter subsytem and let it
>> handle all the issues transparently for us ? I am not sure if all
>> these problems have been addressed in percpu_counter, but would like
>> to know why we are not using percpu_counter for these kinds of things
>> and enhance percpu_counter if it can't handle some of the issues which
>> we are solving here specifically for cpuacct subsystem ?
>>
> At first, generic per-cpu counter sounds interesting but to be honest,
> some special handling is used for cpuacct based on its characteristic.
>

Just trying to understand this clearly ...

>  - Writer works under non-preemptable context.

Which means the writer is running with preemption disabled.
percpu_counter writes (via percpu_counter_add) don't assume anything
and disable preemption themselves. Is this the problem or is there a
bigger issue here why percpu_counter can't be used ?

>  - There is only one writer.

Not sure how you have optimized for this case in cpuacct.
percpu_counters use spinlocks to serialize writers. Are you saying
using spinlocks for this 1 writer case is too much ? Also note that
they update 32 bit percpu counters without any lock and take spinlocks
only when they do a batch update to the 64bit counter.

Regards,
Bharata.
-- 
http://bharata.sulekha.com/blog/posts.htm
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