Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755070AbdDRASi (ORCPT ); Mon, 17 Apr 2017 20:18:38 -0400 Received: from relay8-d.mail.gandi.net ([217.70.183.201]:37858 "EHLO relay8-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753434AbdDRASf (ORCPT ); Mon, 17 Apr 2017 20:18:35 -0400 X-Greylist: delayed 628 seconds by postgrey-1.27 at vger.kernel.org; Mon, 17 Apr 2017 20:18:35 EDT X-Originating-IP: 50.39.160.18 Date: Mon, 17 Apr 2017 17:18:25 -0700 From: Josh Triplett 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, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com Subject: Re: [PATCH v2 tip/core/rcu 04/11] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick Message-ID: <20170418001825.xpnlo46vm5xoo2zc@x> References: <20170417232714.GA19013@linux.vnet.ibm.com> <1492471738-1377-4-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: <1492471738-1377-4-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: NeoMutt/20170306 (1.8.0) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1552 Lines: 35 On Mon, Apr 17, 2017 at 04:28:51PM -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. > > Select a specific number if testing RCU itself. > > - Select the maximum permissible value for large systems. > + Select the maximum permissible value for large systems, but > + please understand that you may also need to set the > + skew_tick kernel boot parameter to avoid contention > + on the rcu_node structure's locks. Nit: the indentation seems wrong here.