From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1946731Ab3BHQrB (ORCPT ); Fri, 8 Feb 2013 11:47:01 -0500 Received: from e23smtp03.au.ibm.com ([202.81.31.145]:56319 "EHLO e23smtp03.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1946657Ab3BHQq5 (ORCPT ); Fri, 8 Feb 2013 11:46:57 -0500 Message-ID: <51152B81.2050501@linux.vnet.ibm.com> Date: Fri, 08 Feb 2013 22:14:49 +0530 From: "Srivatsa S. Bhat" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120828 Thunderbird/15.0 MIME-Version: 1.0 To: Russell King - ARM Linux CC: linux-doc@vger.kernel.org, peterz@infradead.org, fweisbec@gmail.com, linux-kernel@vger.kernel.org, walken@google.com, mingo@kernel.org, linux-arch@vger.kernel.org, xiaoguangrong@linux.vnet.ibm.com, wangyun@linux.vnet.ibm.com, paulmck@linux.vnet.ibm.com, nikunj@linux.vnet.ibm.com, linux-pm@vger.kernel.org, Rusty Russell , rostedt@goodmis.org, rjw@sisk.pl, namhyung@kernel.org, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org, oleg@redhat.com, sbw@mit.edu, tj@kernel.org, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v5 00/45] CPU hotplug: stop_machine()-free CPU hotplug References: <20130122073210.13822.50434.stgit@srivatsabhat.in.ibm.com> <510FBC01.2030405@linux.vnet.ibm.com> <87haloiwv0.fsf@rustcorp.com.au> <51134596.4080106@linux.vnet.ibm.com> <20130208154113.GV17833@n2100.arm.linux.org.uk> In-Reply-To: <20130208154113.GV17833@n2100.arm.linux.org.uk> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13020816-6102-0000-0000-000002F9DD14 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/08/2013 09:11 PM, Russell King - ARM Linux wrote: > On Thu, Feb 07, 2013 at 11:41:34AM +0530, Srivatsa S. Bhat wrote: >> On 02/07/2013 09:44 AM, Rusty Russell wrote: >>> "Srivatsa S. Bhat" writes: >>>> On 01/22/2013 01:03 PM, Srivatsa S. Bhat wrote: >>>> Avg. latency of 1 CPU offline (ms) [stop-cpu/stop-m/c latency] >>>> >>>> # online CPUs Mainline (with stop-m/c) This patchset (no stop-m/c) >>>> >>>> 8 17.04 7.73 >>>> >>>> 16 18.05 6.44 >>>> >>>> 32 17.31 7.39 >>>> >>>> 64 32.40 9.28 >>>> >>>> 128 98.23 7.35 >>> >>> Nice! >> >> Thank you :-) >> >>> I wonder how the ARM guys feel with their quad-cpu systems... >>> >> >> That would be definitely interesting to know :-) > > That depends what exactly you'd like tested (and how) and whether you'd > like it to be a test-chip based quad core, or an OMAP dual-core SoC. > The effect of stop_machine() doesn't really depend on the CPU architecture used underneath or the platform. It depends only on the _number_ of _logical_ CPUs used. And stop_machine() has 2 noticeable drawbacks: 1. It makes the hotplug operation itself slow 2. and it causes disruptions to the workloads running on the other CPUs by hijacking the entire machine for significant amounts of time. In my experiments (mentioned above), I tried to measure how my patchset improves (reduces) the duration of hotplug (CPU offline) itself. Which is also slightly indicative of the impact it has on the rest of the system. But what would be nice to test, is a setup where the workloads running on the rest of the system are latency-sensitive, and measure the impact of CPU offline on them, with this patchset applied. That would tell us how far is this useful in making CPU hotplug less disruptive on the system. Of course, it would be nice to also see whether we observe any reduction in hotplug duration itself (point 1 above) on ARM platforms with lot of CPUs. [This could potentially speed up suspend/resume, which is used rather heavily on ARM platforms]. The benefits from this patchset over mainline (both in terms of points 1 and 2 above) is expected to increase, with increasing number of CPUs in the system. Regards, Srivatsa S. Bhat