Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756812Ab0LJSLP (ORCPT ); Fri, 10 Dec 2010 13:11:15 -0500 Received: from canuck.infradead.org ([134.117.69.58]:40491 "EHLO canuck.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753129Ab0LJSLO convert rfc822-to-8bit (ORCPT ); Fri, 10 Dec 2010 13:11:14 -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: <20101210175645.GB28263@n2100.arm.linux.org.uk> 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> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Fri, 10 Dec 2010 19:10:54 +0100 Message-ID: <1292004654.13513.38.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: 1985 Lines: 54 On Fri, 2010-12-10 at 17:56 +0000, Russell King - ARM Linux wrote: > On Fri, Dec 10, 2010 at 02:47:46PM +0100, Peter Zijlstra wrote: > > inline void update_rq_clock(struct rq *rq) > > { > > - int cpu = cpu_of(rq); > > - u64 irq_time; > > + s64 delta; > > > > if (rq->skip_clock_update) > > return; > > > > - rq->clock = sched_clock_cpu(cpu); > > - irq_time = irq_time_cpu(cpu); > > - if (rq->clock - irq_time > rq->clock_task) > > - rq->clock_task = rq->clock - irq_time; > > + 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. > In terms of the wrap-around, I don't see this as any different from the > above, as: > > rq->clock += sched_clock_cpu(cpu_of(rq)) - rq_clock; > rq->clock = rq->clock + sched_clock_cpu(cpu_of(rq)) - rq_clock; > rq->clock = sched_clock_cpu(cpu_of(rq)); Correct, its not different. Nor was it meant to be. The only problem it solves is the u64 wrap failure in: if (rq->clock - irq_time > rq->clock_task) There are lots of places in the scheduler that rely on u64 wrap, for now the easiest thing for ARM would be to select HAVE_UNSTABLE_SCHED_CLOCK for those platforms that implement a short sched_clock(). While that isn't ideal it is something that makes it work, we can work on something more suitable for future kernels. -- 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/