From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932891AbcGOLor (ORCPT ); Fri, 15 Jul 2016 07:44:47 -0400 Received: from foss.arm.com ([217.140.101.70]:48758 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932656AbcGOLoj (ORCPT ); Fri, 15 Jul 2016 07:44:39 -0400 Date: Fri, 15 Jul 2016 12:46:17 +0100 From: Morten Rasmussen To: Vincent Guittot Cc: Dietmar Eggemann , Peter Zijlstra , "mingo@redhat.com" , Yuyang Du , mgalbraith@suse.de, linux-kernel Subject: Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root domain Message-ID: <20160715114615.GG21816@e105550-lin.cambridge.arm.com> References: <1466615004-3503-1-git-send-email-morten.rasmussen@arm.com> <1466615004-3503-7-git-send-email-morten.rasmussen@arm.com> <578646A8.1070607@arm.com> <20160713163723.GC21816@e105550-lin.cambridge.arm.com> <20160714151518.GD21816@e105550-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160714151518.GD21816@e105550-lin.cambridge.arm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 14, 2016 at 04:15:20PM +0100, Morten Rasmussen wrote: > On Thu, Jul 14, 2016 at 03:25:36PM +0200, Vincent Guittot wrote: > > On 13 July 2016 at 18:37, Morten Rasmussen wrote: > > > Also, for SMT max capacity is less than 1024 already. No? > > > > Yes, it is. I haven't looked in details but i think that we could use > > a capacity of 1024 for SMT with changes that have been done on how to > > evaluate if a sched_group is overloaded or not. > > Changing SMT is a bit more invasive that I had hoped for for this patch > set. I will see if we can make it work with the current SMT capacities. > > > > > > But we may be able to cater for this in wake_cap() somehow. I can have a > > > look if Vincent doesn't like this patch. > > > > IMO, rd->max_cpu_capacity field doesn't seem to be required for now . > > No problem. I will try to get rid of it. I will drop the "arm:" patches > as well as they would have to be extended to guarantee a max capacity of > 1024 and we most likely will have to change it again when Juri's DT > solution hopefully gets merged. I have had a closer look at wake_cap() again. Getting rid of rd->max_cpu_capacity isn't as easy as I thought. The fundamental problem is that all we have in wake_cap() is the waking cpu and previous cpu ids which isn't sufficient to determine whether we have an asymmetric capacity system or not. A capacity <1024 can either a little cpu or an SMT thread. We need a third piece of information, which can be either the highest cpu capacity available in the cpu, or a flag/variable/function telling us whether we are on an SMT system. I see the following solutions to the problem: 1. Have a system-wide max_cpu_capacity (as proposed in this patch) which can let us detect SMT systems as max_cpu_capacity < 1024 implies SMT. 2. Change SMT thread capacity to 1024 so we implicitly know that max capacity is always 1024. As said above, this is a very invasive change as it would mean that we no longer distinguish between SMP and SMT. smt_gain and SD_SHARE_CPUCAPACITY would no longer have any effect and can be ripped out. I would prefer not create a dependency on such a massive change. We can do the experiment afterwards if needed. 3. Detect SMT in wake_cap(). This requires access to the sched_domain hierarchy as the SD_SHARE_CPUCAPACITY is the only way to detect SMT, AFAIK, apart from looping through the capacities of all cpus in the system basically computing max_cpu_capacity each time. wake_cap() is currently called before rcu_read_lock() that gives us access to the sched_domain hierarchy. I would have to postpone the wake_cap() call to being inside the lock and introduce another lookup in the sched_domain hierarchy which would be executed on every wake-up on all systems. IMHO, that is a bit ugly. I don't really like any of the solutions, but of those three I would go for the current solution (1) as it is very minimal both in the amount of code touched/affected and overhead. We can kill it later if we have a better one, no problem for me. Do you see any alternatives?