linux-kernel - Re: [PATCH] percpu_counter : add percpu_counter_add

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Message-Id: <20101021153719.c8bde251.akpm@linux-foundation.org>
Date:	Thu, 21 Oct 2010 15:37:19 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	Dave Chinner <david@...morbit.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: [PATCH] percpu_counter : add percpu_counter_add_fast()

On Sat, 16 Oct 2010 16:19:14 +0200
Eric Dumazet <eric.dumazet@...il.com> wrote:

> The current way to change a percpu_counter is to call
> percpu_counter_add(), which is a bit expensive.
> (More than 40 instructions, possible false sharing, ...)
> 
> When we dont need to maintain the approximate value of the
> percpu_counter (aka fbc->count), and dont need a "s64" wide counter but
> a regular "int" or "long" one, we can use this new function : 
> percpu_counter_add_fast()
> 
> This function is pretty fast : 
> - One instruction on x86 SMP, no register pressure.
> - Is safe in preempt enabled contexts.
> - No lock acquisition, less false sharing.
> 
> Users of this percpu_counter variant should not use
> percpu_counter_read() or percpu_counter_read_positive() anymore, only
> percpu_counter_sum{_positive}() variant.

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);
	if (unlikely(abs(val) > fbc->batch))
		out_of_line_stuff();
}	

I suppose what you're proposing here is useful, although the name isn't
a good one.  It's a different way of using the existing data structure.
I'd suggest that a better name is something like percpu_counter_add_local()?

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