From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755175Ab1I1TKA (ORCPT ); Wed, 28 Sep 2011 15:10:00 -0400 Received: from merlin.infradead.org ([205.233.59.134]:47511 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753298Ab1I1TJ7 convert rfc822-to-8bit (ORCPT ); Wed, 28 Sep 2011 15:09:59 -0400 Subject: Re: [RFD 1/9] Change cpustat fields to an array. From: Peter Zijlstra To: Glauber Costa Cc: linux-kernel@vger.kernel.org, paul@paulmenage.org, lizf@cn.fujitsu.com, daniel.lezcano@free.fr, jbottomley@parallels.com Date: Wed, 28 Sep 2011 21:09:17 +0200 In-Reply-To: <4E836546.7010004@parallels.com> References: <1316816432-9237-1-git-send-email-glommer@parallels.com> <1316816432-9237-2-git-send-email-glommer@parallels.com> <1317157252.21836.3.camel@twins> <4E836546.7010004@parallels.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT X-Mailer: Evolution 3.0.3- Message-ID: <1317236957.24040.62.camel@twins> Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2011-09-28 at 15:19 -0300, Glauber Costa wrote: > On 09/27/2011 06:00 PM, Peter Zijlstra wrote: > > On Fri, 2011-09-23 at 19:20 -0300, Glauber Costa wrote: > >> /* Must have preemption disabled for this to be meaningful. */ > >> -#define kstat_this_cpu __get_cpu_var(kstat) > >> +#define kstat_this_cpu this_cpu_ptr(task_group_kstat(current)) > > > > This just lost you a debug check, the former would whinge when called > > without preemption, the new one wont. Its part of the this_cpu feature > > set to make debugging impossible. > > > >> +#else > >> +#define kstat_cpu(cpu) per_cpu(kstat, cpu) > >> +#define kstat_this_cpu (&__get_cpu_var(kstat)) > >> +#endif > >> > >> extern unsigned long long nr_context_switches(void); > >> > >> @@ -52,8 +62,8 @@ struct irq_desc; > >> static inline void kstat_incr_irqs_this_cpu(unsigned int irq, > >> struct irq_desc *desc) > >> { > >> - __this_cpu_inc(kstat.irqs[irq]); > >> - __this_cpu_inc(kstat.irqs_sum); > >> + kstat_this_cpu->irqs[irq]++; > >> + kstat_this_cpu->irqs_sum++; > > > > It might be worth looking at the asm output of that, I think you made it > > worse, but I'm not quite sure how smart gcc is, it might just figure out > > what you meant. > > I'd say leave it alone. > The biggest difference is that we don't have access to task_group(), or > any of the fields in struct task_group. Because of that, we end up > having to export a function to do the job of dealing with it. > > Users inside sched.c won't have this problem. Outside of it, we'll add a > call to some paths. True, mostly handle_irq paths, but I don't think > that's what's going to kill us. > > Now if we really really want to save it, we'd have to move struct > task_group and its friends to a more visible location like a header... I'm not quite getting how task_group is relevant here. The above will do something like: mov gs:$per-cpu-offset-of-kstat, reg inc reg + idx*8 whereas __this_cpu_inc() could end up like: inc gs:$per-cpu-offset-of-kstat + idx*8 or whatnot. Now clearly gcc could be smart and optimize the temporary reg thing away in the earlier case, or it might not, I really don't know how smart that thing is.