From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752826AbdDMQUC (ORCPT ); Thu, 13 Apr 2017 12:20:02 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:47910 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751624AbdDMQT7 (ORCPT ); Thu, 13 Apr 2017 12:19:59 -0400 Date: Thu, 13 Apr 2017 18:19:48 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick Message-ID: <20170413161948.ymvzlzhporgmldvn@hirez.programming.kicks-ass.net> References: <20170412165441.GA17149@linux.vnet.ibm.com> <1492016149-18834-4-git-send-email-paulmck@linux.vnet.ibm.com> <20170413091535.r6iw7s3pc2znvl6b@hirez.programming.kicks-ass.net> <20170413160332.GZ3956@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170413160332.GZ3956@linux.vnet.ibm.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 13, 2017 at 09:03:33AM -0700, Paul E. McKenney wrote: > On Thu, Apr 13, 2017 at 11:15:35AM +0200, Peter Zijlstra wrote: > > On Wed, Apr 12, 2017 at 09:55:40AM -0700, Paul E. McKenney wrote: > > > If you set RCU_FANOUT_LEAF too high, you can get lock contention > > > on the leaf rcu_node, and you should boot with the skew_tick kernel > > > parameter set in order to avoid this lock contention. This commit > > > therefore upgrades the RCU_FANOUT_LEAF help text to explicitly state > > > this. > > > > > > Signed-off-by: Paul E. McKenney > > > --- > > > init/Kconfig | 10 ++++++++-- > > > 1 file changed, 8 insertions(+), 2 deletions(-) > > > > > > diff --git a/init/Kconfig b/init/Kconfig > > > index a92f27da4a27..946e561e67b7 100644 > > > --- a/init/Kconfig > > > +++ b/init/Kconfig > > > @@ -612,11 +612,17 @@ config RCU_FANOUT_LEAF > > > initialization. These systems tend to run CPU-bound, and thus > > > are not helped by synchronized interrupts, and thus tend to > > > skew them, which reduces lock contention enough that large > > > - leaf-level fanouts work well. > > > + leaf-level fanouts work well. That said, setting leaf-level > > > + fanout to a large number will likely cause problematic > > > + lock contention on the leaf-level rcu_node structures unless > > > + you boot with the skew_tick kernel parameter. > > > > Why mention a way out of a problem you shouldn't have to begin with? > > > > Just state its bad and result in lock contention and leave it at that. > > To avoid people tuning huge machines having to wait for me to give > them an answer as to why they are suffering lock contention after > cranking up the value of RCU_FANOUT_LEAF. > > Or am I missing your point? Your answer should be: don't do that then. Not provide them a shady work around. tick skew isn't pretty and has other problems (there's a reason its not on by default). You're then doing two things you shouldn't.