Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756150AbdDMJPr (ORCPT ); Thu, 13 Apr 2017 05:15:47 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:45275 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750773AbdDMJPp (ORCPT ); Thu, 13 Apr 2017 05:15:45 -0400 Date: Thu, 13 Apr 2017 11:15:35 +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: <20170413091535.r6iw7s3pc2znvl6b@hirez.programming.kicks-ass.net> References: <20170412165441.GA17149@linux.vnet.ibm.com> <1492016149-18834-4-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1492016149-18834-4-git-send-email-paulmck@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 Content-Length: 1308 Lines: 29 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.