From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756861AbYGYX1T (ORCPT ); Fri, 25 Jul 2008 19:27:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756208AbYGYX1A (ORCPT ); Fri, 25 Jul 2008 19:27:00 -0400 Received: from gw.goop.org ([64.81.55.164]:43794 "EHLO mail.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752183AbYGYX07 (ORCPT ); Fri, 25 Jul 2008 19:26:59 -0400 Message-ID: <488A613E.50305@goop.org> Date: Fri, 25 Jul 2008 16:26:54 -0700 From: Jeremy Fitzhardinge User-Agent: Thunderbird 2.0.0.14 (X11/20080501) MIME-Version: 1.0 To: Mike Travis CC: Ingo Molnar , Andrew Morton , "Eric W. Biederman" , Hugh Dickins , Jack Steiner , "H. Peter Anvin" , linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/4] x86_64: Optimize percpu accesses References: <20080725211117.586723000@polaris-admin.engr.sgi.com> In-Reply-To: <20080725211117.586723000@polaris-admin.engr.sgi.com> X-Enigmail-Version: 0.95.6 Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Mike Travis wrote: > This patchset provides the following: > > * x86_64: Cleanup setup_percpu by fixing some minor potential > problems as well as add some debugging aids. > > * x86_64: Rebase per cpu variables to zero > > Rebase per cpu variables to zero in preparation for the following > patch to fold the pda into the per cpu area. > > * x86_64: Fold pda into per cpu area > > Declare the pda as a per cpu variable. This will allow the per cpu > variables to be accessible on the x86_64 using %gs as the base of > the percpu areas for each cpu: > > %gs:per_cpu_xxxx > > * x86_64: Reference zero-based percpu variables offset from gs > > Actually implement the above operation for __get_cpu_var() and > __put_cpu_var(). Since this is now a single instruction, we > can remove the non-preemptible versions of x86_read_percpu() > and x86_write_percpu(). > No, I think you've misunderstood these calls. get_cpu_var(x) evaluates to an lvalue of this cpu's 'x'. It disables preemption, in the same manner as get_cpu(). put_cpu_var(x) does nothing more than re-enable preemption, to pair with get_cpu_var(). __get_cpu_var(x) is the same as get_cpu_var, but it assumes that preemption is already disabled. There is no __put_cpu_var(). The important point is that an expression like "__get_cpu_var(x) = foo" does not evaluate to a single instruction, and is not preempt or interrupt -atomic. That's the reason x86_X_percpu() exist, since they're a single instruction in an asm. However, with %gs: based addressing they can be easily unified. J