From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755919Ab3BVIq2 (ORCPT ); Fri, 22 Feb 2013 03:46:28 -0500 Received: from 173-166-109-252-newengland.hfc.comcastbusiness.net ([173.166.109.252]:53854 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755219Ab3BVIqY (ORCPT ); Fri, 22 Feb 2013 03:46:24 -0500 Message-ID: <1361522767.26780.44.camel@laptop> Subject: Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats From: Peter Zijlstra To: Kevin Hilman Cc: Frederic Weisbecker , Russell King , Thomas Gleixner , Steven Rostedt , Ingo Molnar , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linaro-kernel@lists.linaro.org Date: Fri, 22 Feb 2013 09:46:07 +0100 In-Reply-To: <1361512604-2720-1-git-send-email-khilman@linaro.org> References: <1361512604-2720-1-git-send-email-khilman@linaro.org> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.6.2-0ubuntu0.1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2013-02-21 at 21:56 -0800, Kevin Hilman wrote: > On 64-bit platforms, reads/writes of the various cpustat fields are > atomic due to native 64-bit loads/stores. However, on non 64-bit > platforms, reads/writes of the cpustat fields are not atomic and could > lead to inconsistent statistics. Which is a problem how? > This problem was originally reported by Frederic Weisbecker as a > 64-bit limitation with the nsec granularity cputime accounting for > full dynticks, but then we realized that it's a problem that's been > around for awhile and not specific to the new cputime accounting. > > This series fixes this by first converting all access to the cputime > fields to use accessor functions, and then converting the accessor > functions to use the atomic64 functions. Argh!! at what cost? 64bit atomics are like expensive. Wouldn't adding a seqlock be saner?