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

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Message-Id: <20101021154333.f67fd870.akpm@linux-foundation.org>
Date:	Thu, 21 Oct 2010 15:43:33 -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, ...)

This is incorrect.  With my compiler it's 25 instructions except in the
very rare case where a batch overflow occurs.

And more than half of that is function call entry/exit overhead.

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