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

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Message-ID: <1305268456.2831.38.camel@edumazet-laptop>
Date:	Fri, 13 May 2011 08:34:16 +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 13:28 +0800, Shaohua Li a écrit :
> Hi,
> On Fri, May 13, 2011 at 01:20:06PM +0800, Eric Dumazet wrote:
> > 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.
> I thought your proposal is:
> in _add
> {
> 	if (count >= batch || count <= -batch) {
> 		fbc->seq_count++;
>                atomic64_add(count, &fbc->count);
> -------->
>                 __this_cpu_write(*fbc->counters, 0);
> 	}
> }
> 
> in _sum
> {
> restart:
> 	oldseq = fbc->seqcount;
> 	smp_rmb();
> 	do_sum();
> 	smp_rmb()
> 	newseq = fbc->seqcount;
> 	if (newseq - oldseq >= maxfuzzy)
> 		goto restart;
> 	return ret;
> } 
> if _sum run between above line marked in _add, then the seqcount check
> doesn't work, we still have deviation Tejun pointed out.
> 

I see the point thanks, I'll think a bit more about it.

We currently serializes both _sum() and _add() with a spinlock.

My idea was OK if we still kept spinlock in _add(), but this obviously
is not the need.

Your goal is letting _add() run without spinlock, but can we agree
_sum() can run with a spinlock() like today [no more than one instance
of _sum() running per percpu_counter] ?



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