From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753495AbaFMVKp (ORCPT ); Fri, 13 Jun 2014 17:10:45 -0400 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:34418 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751262AbaFMVKo (ORCPT ); Fri, 13 Jun 2014 17:10:44 -0400 X-Originating-IP: 173.246.103.110 Date: Fri, 13 Jun 2014 14:10:35 -0700 From: Josh Triplett To: "Paul E. McKenney" Cc: Frederic Weisbecker , LKML , Steven Rostedt , Mathieu Desnoyers Subject: Re: [PATCH] rcu: Only pin GP kthread when full dynticks is actually used Message-ID: <20140613211034.GA10651@jtriplet-mobl1> References: <1402618619-32630-1-git-send-email-fweisbec@gmail.com> <20140613012432.GH4581@linux.vnet.ibm.com> <20140613013515.GA9589@linux.vnet.ibm.com> <20140613124714.GC6635@localhost.localdomain> <20140613155233.GM4581@linux.vnet.ibm.com> <20140613160002.GL6635@localhost.localdomain> <20140613161630.GQ4581@linux.vnet.ibm.com> <20140613162130.GP6635@localhost.localdomain> <20140613164441.GA14232@thin> <20140613204822.GT4581@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140613204822.GT4581@linux.vnet.ibm.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 On Fri, Jun 13, 2014 at 01:48:22PM -0700, Paul E. McKenney wrote: > On Fri, Jun 13, 2014 at 09:44:41AM -0700, Josh Triplett wrote: > > On Fri, Jun 13, 2014 at 06:21:32PM +0200, Frederic Weisbecker wrote: > > > On Fri, Jun 13, 2014 at 09:16:30AM -0700, Paul E. McKenney wrote: > > > > > Is it because we have dynticks CPUs staying too long in the kernel without > > > > > taking any quiescent states? Are we perhaps missing some rcu_user_enter() or > > > > > things? > > > > > > > > Sort of the former, but combined with the fact that in-kernel CPUs still > > > > need scheduling-clock interrupts for RCU to make progress. I could > > > > move this to RCU's context-switch hook, but that could be very bad for > > > > workloads that do lots of context switching. > > > > > > Or I can restart the tick if the CPU stays in the kernel for too long without > > > a tick. I think that's what we were doing before but we removed that because > > > we never implemented it correctly (we sent scheduler IPI that did nothing...) > > > > I wonder if timer slack would make sense here: when you have at least > > one RCU callback pending, set a timer with a huge amount of timer slack, > > and cancel it if you end up handling the callback via a trip through the > > scheduler. > > But in this case, we need the tick even if the current CPU has no callbacks > because it might be in an RCU read-side critical section. Don't we handle that case via the slowpath of rcu_read_unlock, and a flag set via IPI? ("Oh, that CPU has taken too long to note a quiescent state; send it an IPI to set the special flag that makes unlock do the work.") - Josh Triplett