From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756229Ab3BVMu3 (ORCPT ); Fri, 22 Feb 2013 07:50:29 -0500 Received: from mail-ve0-f182.google.com ([209.85.128.182]:45847 "EHLO mail-ve0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755920Ab3BVMu0 (ORCPT ); Fri, 22 Feb 2013 07:50:26 -0500 Date: Fri, 22 Feb 2013 13:50:20 +0100 From: Frederic Weisbecker To: Peter Zijlstra Cc: Kevin Hilman , Russell King , Thomas Gleixner , Steven Rostedt , Ingo Molnar , 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: <20130222125019.GC17948@somewhere.redhat.com> References: <1361512604-2720-1-git-send-email-khilman@linaro.org> <1361522767.26780.44.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1361522767.26780.44.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 On Fri, Feb 22, 2013 at 09:46:07AM +0100, Peter Zijlstra wrote: > 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? So here is a possible scenario, CPU 0 reads a kcpustat value, and CPU 1 writes it at the same time: //Initial value of "cpustat" is 0xffffffff == CPU 0 == == CPU 1 == //load low part mov %eax, [cpustat] inc [cpustat] //Update the high part if necessary jnc 1f inc [cpustat + 4] 1: //load high part mov %edx, [cpustat + 4] Afterward, CPU 0 will think the value is 0x1ffffffff while it's actually 0x100000000. atomic64_read() and atomic64_set() are supposed to take care of that, without even the need for _inc() or _add() parts that use LOCK. > > > 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? Not sure. This requires a spinlock in the write side which is called from fast path like the timer interrupt.