From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756345AbbEUN3S (ORCPT ); Thu, 21 May 2015 09:29:18 -0400 Received: from mail-pd0-f181.google.com ([209.85.192.181]:32889 "EHLO mail-pd0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756324AbbEUN3O (ORCPT ); Thu, 21 May 2015 09:29:14 -0400 Date: Thu, 21 May 2015 18:59:05 +0530 From: Afzal Mohammed To: Frederic Weisbecker Cc: "Paul E. McKenney" , Mike Galbraith , Sasha Levin , Ingo Molnar , LKML , Chris Metcalf , "Rafael J . Wysocki" , Peter Zijlstra , Dave Jones , Thomas Gleixner , Oleg Nesterov , Ingo Molnar , Rik van Riel , Martin Schwidefsky Subject: Re: [PATCH 4/4] nohz: Set isolcpus when nohz_full is set Message-ID: <20150521132905.GA8706@afzalpc> References: <1430928266-24888-1-git-send-email-fweisbec@gmail.com> <1430928266-24888-5-git-send-email-fweisbec@gmail.com> <55579CE0.5060801@gmail.com> <1431840650.3222.78.camel@gmail.com> <20150520203809.GA2940@afzalpc> <20150520210026.GC6776@linux.vnet.ibm.com> <20150521121246.GA4723@afzalpc> <20150521125759.GL6776@linux.vnet.ibm.com> <20150521130621.GA14324@lerouge> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150521130621.GA14324@lerouge> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Thu, May 21, 2015 at 03:06:23PM +0200, Frederic Weisbecker wrote: > On Thu, May 21, 2015 at 05:57:59AM -0700, Paul E. McKenney wrote: > > Indeed, NO_HZ_FULL is special purpose. You normally would select > > NO_HZ_FULL_ALL only on a system intended for heavy compute without > > normal-workload distractions or for some real-time systems. For mixed > > workloads, you would build with NO_HZ_FULL (but not NO_HZ_FULL_ALL) and > > use the boot parameters to select which CPUs are to be running the > > specialized portion of the workload. > > > > And you would of course need to lead enough CPUs running normally to > > handle the non-specialized portion of the workload. > > > > This sort of thing has traditionally required specialized kernels, > > so the cool thing here is that we can make Linux do it. Though, as > > you noticed, careful configuration is still required. > > > > Seem reasonable? Yes, thanks, some dots got connected :) > That said if he saw a big performance regression after applying these patches, > then there is likely a problem in the patchset. Well it could be due to that mode > which loops on full dynticks before resuming to userspace. Indeed when that is > enabled, I expect real throughput issues on workloads doing lots of kernel <-> > userspace roundtrips. We just need to make sure this thing only works when requested. With this change (& having NO_HZ_FULL_ALL), hackbench was being served only by the boot cpu, while w/o this change, all 8 (this is a quad core HT processor) was being used - observation based on 'top'. Regards Afzal