Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751094AbbDYXNR (ORCPT ); Sat, 25 Apr 2015 19:13:17 -0400 Received: from mail-wi0-f171.google.com ([209.85.212.171]:34159 "EHLO mail-wi0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750891AbbDYXNO (ORCPT ); Sat, 25 Apr 2015 19:13:14 -0400 Date: Sun, 26 Apr 2015 01:13:10 +0200 From: Frederic Weisbecker To: Gene Heskett Cc: LKML , Chris Metcalf , "Rafael J . Wysocki" , Peter Zijlstra , Mike Galbraith , Ingo Molnar , Dave Jones , Thomas Gleixner , Oleg Nesterov , "Paul E . McKenney" , Ingo Molnar , Rik van Riel , Martin Schwidefsky Subject: Re: [PATCH 4/4] nohz: Set isolcpus when nohz_full is set Message-ID: <20150425231308.GA2188@lerouge> References: <1429891111-30855-1-git-send-email-fweisbec@gmail.com> <1429891111-30855-5-git-send-email-fweisbec@gmail.com> <201504241607.52460.gheskett@wdtv.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201504241607.52460.gheskett@wdtv.com> 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 Content-Length: 2621 Lines: 53 On Fri, Apr 24, 2015 at 04:07:52PM -0400, Gene Heskett wrote: > On Friday 24 April 2015 11:58:31 Frederic Weisbecker wrote: > > From: Chris Metcalf > > > > nohz_full is only useful with isolcpus also set, since otherwise the > > scheduler has to run periodically to try to determine whether to steal > > work from other cores. > > > > Accordingly, when booting with nohz_full=xxx on the command line, we > > should act as if isolcpus=xxx was also set, and set (or extend) the > > isolcpus set to include the nohz_full cpus. > > > > Acked-by: Mike Galbraith ["thumbs up!"] > > Acked-by: Rik van Riel > > Acked-by: Peter Zijlstra (Intel) > > Signed-off-by: Chris Metcalf > > Cc: Peter Zijlstra (Intel) > > Cc: Paul E. McKenney > > Cc: Rafael J. Wysocki > > Cc: Martin Schwidefsky > > Cc: Mike Galbraith > > Cc: Ingo Molnar > > Cc: Rik van Riel > > Signed-off-by: Frederic Weisbecker > > As a user of LinuxCNC, we expect the core(s) so isolated by the isolcpus > argument at bootup time to remain undisturbed in order to preserve the > I/O updates heartbeat latency at the absolute minimum that board and cpu > combo can accomplish. > > If this patch changes that behaviour such that the isolated core is > grabbed for another job while the RTAI bits and pieces are loaded and > running machinery, causing the machinery to lose this steady, possibly > as little as a 20 microsecond period repeating operation heartbeat, this > will quite effectively destroy our ability to run this software on linux > without farming that whole operation out to intelligent I/O cards. > > Please keep this in mind. I don't read the patch well enough to > determine this myself. I think it's not a problem. This patch isn't adding any work or noise to isolcpus, it's actually setting CPUs that run tickless in userspace to be part of the isolcpus set, because we need userspace-tickless CPUs to not be disturbed at all. You shouldn't be concerned if you don't use full dynticks. If you happen to use full dynticks in your usecase one day, you'll use it on your isolcpus anyway. -- 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/