From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751464AbaFMMrU (ORCPT ); Fri, 13 Jun 2014 08:47:20 -0400 Received: from mail-we0-f181.google.com ([74.125.82.181]:52086 "EHLO mail-we0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750863AbaFMMrT (ORCPT ); Fri, 13 Jun 2014 08:47:19 -0400 Date: Fri, 13 Jun 2014 14:47:16 +0200 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: LKML , Josh Triplett , Steven Rostedt , Mathieu Desnoyers Subject: Re: [PATCH] rcu: Only pin GP kthread when full dynticks is actually used Message-ID: <20140613124714.GC6635@localhost.localdomain> References: <1402618619-32630-1-git-send-email-fweisbec@gmail.com> <20140613012432.GH4581@linux.vnet.ibm.com> <20140613013515.GA9589@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140613013515.GA9589@linux.vnet.ibm.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, Jun 12, 2014 at 06:35:15PM -0700, Paul E. McKenney wrote: > On Thu, Jun 12, 2014 at 06:24:32PM -0700, Paul E. McKenney wrote: > > On Fri, Jun 13, 2014 at 02:16:59AM +0200, Frederic Weisbecker wrote: > > > CONFIG_NO_HZ_FULL may be enabled widely on distros nowadays but actual > > > users should be a tiny minority, if actually any. > > > > > > Also there is a risk that affining the GP kthread to a single CPU could > > > end up noticeably reducing RCU performances and increasing energy > > > consumption. > > > > > > So lets affine the GP kthread only when nohz full is actually used > > > (ie: when the nohz_full= parameter is filled or CONFIG_NO_HZ_FULL_ALL=y) > > Which reminds me... Kernel-heavy workloads running NO_HZ_FULL_ALL=y > can see long RCU grace periods, as in about two seconds each. It is > not hard for me to detect this situation. Ah yeah sounds quite long. > Is there some way I can > call for a given CPU's scheduling-clock interrupt to be turned on? Yeah, once the nohz kick patchset (https://lwn.net/Articles/601214/) is merged, a simple call to tick_nohz_full_kick_cpu() should do the trick. Although the right condition must be there on the IPI side. Maybe with rcu_needs_cpu() or such. But it would be interesting to identify the sources of these extended grace periods. If we only restart the tick, we may ignore some deeper oustanding issue. Thanks. > > I believe that the nsproxy guys were seeing something like this as well. > > Thanx, Paul