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

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Date:	Wed, 4 Mar 2009 17:20:05 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Bharata B Rao <bharata.rao@...il.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, 4 Mar 2009 13:24:43 +0530
Bharata B Rao <bharata.rao@...il.com> wrote:

> On Wed, Mar 4, 2009 at 12:02 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> >
> > cgroup/cpuacct subsystem counts cpu usage by 64bit coutnter in
> > per-cpu object. In read-side (via cpuacct.usage file), for reading 64bit
> > value in safe manner, it takes rq->lock of (other) cpus.
> >
> > In general, taking rq->lock of other cpus from codes not for scheduler
> > is not good. This patch tries to remove rq->lock in read-side.
> >
> > To read 64bit value in atomic, this patch uses seqcounter.
> >
> > Pros.
> >  - rq->lock is not necessary.
> > Cons.
> >  - When updating counter, sequence number must be updated.
> >    (I hope this per-cpu sequence number is on cache...)
> >
> > Changelog: v1->v2
> >  - checking calling context of all calls and avoid unnecessary
> >   preempt_disable calls.
> >  - use on_each_cpu() instead of workqueue, at reset
> >
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> So cpuacct->cpuusage is a 64 bit percpu counter and I see that cpuacct
> subsystem itself handles the following special cases:
> - 32 vs 64 bit update issues
> - resetting percpu counters on online and offline cpus
> 
> Tomorrow if I add other counters to cpuacct subsystem (like stime and
> utime), I need to do what you have done in this patch all over again
> for the additional counters.
> 
I'm not sure Yes or No. Is it necessary to be per-cpu ?
IIUC, stime/utime update is at most once-per-tick and not so frequent
as cpuacct.

> 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.

  - Writer works under non-preemptable context.
  - There is only one writer.

Otherwize, using generic atomic64_t.. or res_counter will be good...maybe.
(res_counter updates 64bit value under spinlock lock.)

Thanks,
-Kame




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

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