Date: Wed, 18 Mar 2009 12:54:34 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: bharata@linux.vnet.ibm.com
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, 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>
Subject: Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in
 cpuacct_charge() safe when rcupreempt is used.
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On Wed, 18 Mar 2009 08:55:58 +0530
Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:

> 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.
> 

Is this wrong ?
==
-- CONFIG_32BIT
s64 static inline s64 percpu_counter_read_slow(struct percpu_counter *fbc)
{
   s64 val;
retry:
    val = fbc->counter;
    smp_mb();
    wait_spin_unlock(&fbc->lock);
    if (fbc->counter < val) {
            goto retry;
    return val;
}
==

Thanks,
-Kame


> Regards,
> Bharata.
> --
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