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Date: Thu, 21 Oct 2010 17:45:36 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Christoph Lameter <cl@linux.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>, Nick Piggin <npiggin@kernel.dk>,
        Dave Chinner <david@fromorbit.com>, linux-fsdevel@vger.kernel.org,
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Subject: Re: [PATCH] percpu_counter : add percpu_counter_add_fast()
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On Thu, 21 Oct 2010 18:10:13 -0500 (CDT) Christoph Lameter <cl@linux.com> wrote:

> On Thu, 21 Oct 2010, Andrew Morton wrote:
> 
> > That isn't actually what I was suggesting.  I was suggesting the use of
> > an inlined, this_cpu_add()-using percpu_counter_add() variant which
> > still does the batched spilling into ->count.  IOW, just speed up the
> > current implementation along the lines of
> >
> > {
> > 	val = this_cpu_add_return(*fbc->counters, amount);
> 
> this_cpu_add_return() is not in the kernel but could be realized using a
> variant offshoot of cmpxchg_local. I had something like that initially but
> omitted it since there was no use case.

this_cpu_add_return() isn't really needed in this application.

{
	this_cpu_add(*fbc->counters, amount);
	if (unlikely(abs(this_cpu_read(*fbc->counters)) > fbc->batch))
		out_of_line_stuff();
}	

will work just fine.