From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755511AbeEaPVU (ORCPT ); Thu, 31 May 2018 11:21:20 -0400 Received: from merlin.infradead.org ([205.233.59.134]:42482 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755423AbeEaPVR (ORCPT ); Thu, 31 May 2018 11:21:17 -0400 Date: Thu, 31 May 2018 17:20:50 +0200 From: Peter Zijlstra To: Waiman Long Cc: Tejun Heo , Li Zefan , Johannes Weiner , Ingo Molnar , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kernel-team@fb.com, pjt@google.com, luto@amacapital.net, Mike Galbraith , torvalds@linux-foundation.org, Roman Gushchin , Juri Lelli , Patrick Bellasi , Thomas Gleixner Subject: Re: [PATCH v9 3/7] cpuset: Add cpuset.sched.load_balance flag to v2 Message-ID: <20180531152050.GK12180@hirez.programming.kicks-ass.net> References: <1527601294-3444-1-git-send-email-longman@redhat.com> <1527601294-3444-4-git-send-email-longman@redhat.com> <20180531122638.GJ12180@hirez.programming.kicks-ass.net> <42cc1f44-2355-1c0c-b575-49c863303c42@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <42cc1f44-2355-1c0c-b575-49c863303c42@redhat.com> User-Agent: Mutt/1.9.5 (2018-04-13) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 31, 2018 at 09:54:27AM -0400, Waiman Long wrote: > On 05/31/2018 08:26 AM, Peter Zijlstra wrote: > > I still find all that a bit weird. > > > > So load_balance=0 basically changes a partition into a > > 'fully-partitioned partition' with the seemingly random side-effect that > > now sub-partitions are allowed to consume all CPUs. > > Are you suggesting that we should allow sub-partition to consume all the > CPUs no matter the load balance state? I can live with that if you think > it is more logical. I'm on the fence myself; the only thing I'm fairly sure of is that tying this particular behaviour to the load-balance knob seems off. > > The rationale, only given in the Changelog above, seems to be to allow > > 'easy' emulation of isolcpus. > > > > I'm still not convinced this is a useful knob to have. You can do > > fully-partitioned by simply creating a lot of 1 cpu parititions. > > That is certainly true. However, I think there are some additional > overhead in the scheduler side in maintaining those 1-cpu partitions. Right? cpuset-controller as such doesn't have much overhead scheduler wise, cpu-controller OTOH does, and there depth is the predominant factor, so many sibling groups should not matter there either. > > So this one knob does two separate things, both of which seem, to me, > > redundant. > > > > Can we please get better rationale for this? > > I am fine getting rid of the load_balance flag if this is the consensus. > However, we do need to come up with a good migration story for those > users that need the isolcpus capability. I think Mike was the one asking > for supporting isolcpus. So Mike, what is your take on that. So I don't strictly mind having a knob that does the 'fully-partitioned partition' thing -- however odd that sounds -- but I feel we should have a solid use-case for it. I also think we should not mix the 'consume all' thing with the 'fully-partitioned' thing, as they are otherwise unrelated.