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Date: Wed, 18 Mar 2009 08:55:58 +0530
From: Bharata B Rao <bharata@linux.vnet.ibm.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: balbir@linux.vnet.ibm.com, Li Zefan <lizf@cn.fujitsu.com>,
       linux-kernel@vger.kernel.org, Dhaval Giani <dhaval@linux.vnet.ibm.com>,
       Paul Menage <menage@google.com>, Ingo Molnar <mingo@elte.hu>,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Subject: Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in
	cpuacct_charge() safe when rcupreempt is used.
Message-ID: <20090318032558.GB3960@in.ibm.com>
Reply-To: bharata@linux.vnet.ibm.com
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On Tue, Mar 17, 2009 at 03:04:46PM +0100, Peter Zijlstra wrote:
> On Tue, 2009-03-17 at 19:29 +0530, Balbir Singh wrote:
> > * Peter Zijlstra <a.p.zijlstra@chello.nl> [2009-03-17 14:26:01]:
> > 
> > > On Tue, 2009-03-17 at 18:42 +0530, Balbir Singh wrote:
> > > 
> > > > I'd like to get the patches in -tip and see the results, I would
> > > > recommend using percpu_counter_sum() while reading the data as an
> > > > enhancement to this patch. If user space does not overwhelm with a lot
> > > > of reads, sum would work out better.
> > > 
> > > You trust userspace? I'd rather not.
> > >
> > 
> > Fair enough.. A badly written application monitor can frequently read
> > this data and cause horrible performance issues. On the other hand
> > large number of CPUs can make the lag even worse. Is it time yet for
> > percpu_counter batch numbers? I've tested this patch and the results
> > were not badly off the mark. 
> 
> I'd rather err on the side of caution here, you might get some crazy
> folks depending on it and then expecting us to maintain it.

So if we want to be cautious, we could use percpu_counter_sum() as
Balbir suggested. That would address both the issues with percpu_counter
that I pointed out earlier:

- Readers are serialized with writers and we get consistent/correct
  values during reads.
- Negates the effect of batching and reads would always get updated/current
  values.

Regards,
Bharata.