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

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Date:	Fri, 13 May 2011 11:09:03 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	"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

Hi,
On Thu, May 12, 2011 at 05:05:34PM +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.
there is another problem here, _sum could keep spin if cocurrent updater
is running.

We could slightly change Eric's idea, how about something like this:

s64 __percpu_counter_sum(struct percpu_counter *fbc)
{
	retry_times = 0;
retry:
	old_seq = fbc->seq;
	sum = do_sum()
	if (old_seq != fbc->seq && retry_times++ < MAX_RETRY)
		goto retry;
	return sum;
}
MAX_RETRY could be nr_cpu_ids. The rational here is if cocurrent updater
keeps running, we can't get accurate sum, so just don't try hard.

Thanks,
Shaohua
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