From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756685Ab3BVOSU (ORCPT ); Fri, 22 Feb 2013 09:18:20 -0500 Received: from mail-ee0-f41.google.com ([74.125.83.41]:51032 "EHLO mail-ee0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755376Ab3BVOSO (ORCPT ); Fri, 22 Feb 2013 09:18:14 -0500 Date: Fri, 22 Feb 2013 15:18:10 +0100 From: Ingo Molnar To: Peter Zijlstra Cc: Frederic Weisbecker , Kevin Hilman , Russell King , Thomas Gleixner , Steven Rostedt , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linaro-kernel@lists.linaro.org Subject: Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats Message-ID: <20130222141810.GB9606@gmail.com> References: <1361512604-2720-1-git-send-email-khilman@linaro.org> <1361522767.26780.44.camel@laptop> <20130222125019.GC17948@somewhere.redhat.com> <1361542150.26780.64.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1361542150.26780.64.camel@laptop> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Peter Zijlstra wrote: > On Fri, 2013-02-22 at 13:50 +0100, Frederic Weisbecker wrote: > > > Argh!! at what cost? 64bit atomics are like expensive. Wouldn't > > adding > > > a seqlock be saner? > > > > Not sure. This requires a spinlock in the write side which is called > > from > > fast path like the timer interrupt. > > A single spinlock is _way_ cheaper than a ton of cmpxchg8b()s > to update a few variables. Every cmpxchg8b() will be roughly as expensive as a spinlock acquire+release fastpath. Thanks, Ingo