linux-kernel - Re: [patch v2 0/5] percpu_counter: bug fix and enhancement

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Message-ID: <1305264007.2831.14.camel@edumazet-laptop>
Date:	Fri, 13 May 2011 07:20:06 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"cl@...ux.com" <cl@...ux.com>,
	"npiggin@...nel.dk" <npiggin@...nel.dk>
Subject: Re: [patch v2 0/5] percpu_counter: bug fix and enhancement

Le vendredi 13 mai 2011 à 12:37 +0800, Shaohua Li a écrit :
> On Thu, 2011-05-12 at 17:05 +0800, Tejun Heo wrote:
> > Hello,
> > 
> > On Thu, May 12, 2011 at 11:02:15AM +0200, Eric Dumazet wrote:
> > > > I don't think @maxfuzzy is necessary there.  I wrote this before but
> > > > why can't we track the actual deviation instead of the number of
> > > > deviation events?
> > > 
> > > Thats roughly same thing (BATCH multiplicator factor apart)
> > > 
> > > Most percpu_counter users for a given percpu_counter object use a given
> > > BATCH, dont they ?
> > 
> > Well, @maxfuzzy is much harder than @batch.  It's way less intuitive.
> > Although I haven't really thought about it that much, I think it might
> > be possible to eliminate it.  Maybe I'm confused.  I'll take another
> > look later but if someone can think of something, please jump right
> > in.
> Hmm, looks Eric's approach doesn't work. because we want to remove lock
> in _add, checking seq in _sum still races with _add.
> 

Why ?

I'll code a patch, I believe it should work.

A seqcount is not a 'lock'.

The thing is we want _add to be real fast, so it must not hit a lock set
in _sum()

[Think about a machine with 4096 cpus]



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