Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753493AbbBXKWT (ORCPT ); Tue, 24 Feb 2015 05:22:19 -0500 Received: from mail-oi0-f53.google.com ([209.85.218.53]:35789 "EHLO mail-oi0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753472AbbBXKWP (ORCPT ); Tue, 24 Feb 2015 05:22:15 -0500 MIME-Version: 1.0 In-Reply-To: <20150219171824.GC22059@e105550-lin.cambridge.arm.com> References: <1421316570-23097-1-git-send-email-vincent.guittot@linaro.org> <1421316570-23097-6-git-send-email-vincent.guittot@linaro.org> <20150219165241.GL5029@twins.programming.kicks-ass.net> <20150219171824.GC22059@e105550-lin.cambridge.arm.com> From: Vincent Guittot Date: Tue, 24 Feb 2015 11:21:54 +0100 Message-ID: Subject: Re: [PATCH RESEND v9 05/10] sched: make scale_rt invariant with frequency To: Morten Rasmussen Cc: Peter Zijlstra , "mingo@kernel.org" , "linux-kernel@vger.kernel.org" , "preeti@linux.vnet.ibm.com" , "kamalesh@linux.vnet.ibm.com" , "riel@redhat.com" , "efault@gmx.de" , "nicolas.pitre@linaro.org" , Dietmar Eggemann , "linaro-kernel@lists.linaro.org" Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3196 Lines: 67 On 19 February 2015 at 18:18, Morten Rasmussen wrote: > On Thu, Feb 19, 2015 at 04:52:41PM +0000, Peter Zijlstra wrote: >> On Thu, Jan 15, 2015 at 11:09:25AM +0100, Vincent Guittot wrote: >> > The average running time of RT tasks is used to estimate the remaining compute >> > capacity for CFS tasks. This remaining capacity is the original capacity scaled >> > down by a factor (aka scale_rt_capacity). This estimation of available capacity >> > must also be invariant with frequency scaling. >> > >> > A frequency scaling factor is applied on the running time of the RT tasks for >> > computing scale_rt_capacity. >> > >> > In sched_rt_avg_update, we scale the RT execution time like below: >> > rq->rt_avg += rt_delta * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT >> > >> > Then, scale_rt_capacity can be summarized by: >> > scale_rt_capacity = SCHED_CAPACITY_SCALE - >> > ((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period) >> > >> > We can optimize by removing right and left shift in the computation of rq->rt_avg >> > and scale_rt_capacity >> >> So far so good.. >> >> > The call to arch_scale_frequency_capacity in the rt scheduling path might be >> > a concern for RT folks because I'm not sure whether we can rely on >> > arch_scale_freq_capacity to be short and efficient ? >> >> No, that is, arch_scale_frequency_capacity() _must_ be short and >> efficient, event for the fair class, its called in very hot paths. > > ... and very frequently too. > >> I think we've talked about this before; this function should basically >> only return a cached value, which is periodically updated through some >> means. > > Agreed. I think it is reasonable to assume that the arch code > implementing arch_scale_freq_capacity() does it's best to make it fast > for the particular architecture. Since the scaling factor to be returned > by the function may be obtained in different ways for different > architectures the caching should be done on the arch side. > >> But lets see, I've yet to see an actual implementation of it; and its >> got that sd argument, curious what you're going to do with that. > > So we do have an RFC implementation for ARM already which I posted in > December and is also included in the rather large RFC posting I did some > weeks ago. That one basically reads two atomic variables and returns the > ratio between the two. I have yet to benchmark how horribly expensive it > is though. The sd argument is ignored. We might actually not need it at > all? For consistency across all arch_scale_xx_capacity, i would prefer to keep the same prototype interface (struct sched_domain *sd, int cpu) even if it's not used ofr now > -- > 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/ -- 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/