Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756756Ab0LJT0S (ORCPT ); Fri, 10 Dec 2010 14:26:18 -0500 Received: from casper.infradead.org ([85.118.1.10]:33923 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752114Ab0LJT0R convert rfc822-to-8bit (ORCPT ); Fri, 10 Dec 2010 14:26:17 -0500 Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM From: Peter Zijlstra To: Russell King - ARM Linux Cc: Venkatesh Pallipadi , Mikael Pettersson , Ingo Molnar , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, John Stultz In-Reply-To: <1292004654.13513.38.camel@laptop> References: <1291917330.6803.7.camel@twins> <1291920939.6803.38.camel@twins> <1291936593.13513.3.camel@laptop> <1291975704.6803.59.camel@twins> <1291987065.6803.151.camel@twins> <1291987635.6803.161.camel@twins> <1291988866.6803.171.camel@twins> <20101210175645.GB28263@n2100.arm.linux.org.uk> <1292004654.13513.38.camel@laptop> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Fri, 10 Dec 2010 20:25:56 +0100 Message-ID: <1292009156.13513.47.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1775 Lines: 52 On Fri, 2010-12-10 at 19:10 +0100, Peter Zijlstra wrote: > > > + delta = sched_clock_cpu(cpu_of(rq)) - rq->clock; > > > + rq->clock += delta; > > > > Hmm. Can you tell me how this is different to: > > > > new_clock = sched_clock_cpu(cpu_of(rq)); > > delta = new_clock - rq->clock; > > rq->clock = new_clock; > > > > which I think may be simpler in terms of 64-bit math for 32-bit compilers > > to deal with? > > Its not, I could write it like that, the only reason I didn't is because > it uses an extra variable. If gcc on 32bit targets really generates > hideous code for it I'll happily change it. Index: linux-2.6/kernel/sched.c =================================================================== --- linux-2.6.orig/kernel/sched.c +++ linux-2.6/kernel/sched.c @@ -1863,6 +1863,7 @@ void account_system_vtime(struct task_st { unsigned long flags; s64 delta; + u64 now; int cpu; if (!sched_clock_irqtime) @@ -1870,8 +1871,9 @@ void account_system_vtime(struct task_st local_irq_save(flags); cpu = smp_processor_id(); - delta = sched_clock_cpu(cpu) - per_cpu(irq_start_time, cpu); - per_cpu(irq_start_time, cpu) += delta; + now = sched_clock_cpu(cpu); + delta = now - per_cpu(irq_start_time, cpu); + per_cpu(irq_start_time, cpu) = now; irq_time_write_begin(cpu); On i386 (gcc (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5): Before: account_system_vtime: 160 bytes After: account_system_vtime: 214 bytes -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/