From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753802AbdDRSmS (ORCPT ); Tue, 18 Apr 2017 14:42:18 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:34326 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750954AbdDRSmP (ORCPT ); Tue, 18 Apr 2017 14:42:15 -0400 Date: Tue, 18 Apr 2017 11:42:07 -0700 From: "Paul E. McKenney" To: Josh Triplett 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 Reply-To: paulmck@linux.vnet.ibm.com References: <20170417232714.GA19013@linux.vnet.ibm.com> <1492471738-1377-4-git-send-email-paulmck@linux.vnet.ibm.com> <20170418001825.xpnlo46vm5xoo2zc@x> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170418001825.xpnlo46vm5xoo2zc@x> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17041818-0024-0000-0000-0000024C683B X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006936; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00849246; UDB=6.00419324; IPR=6.00627886; BA=6.00005298; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015087; XFM=3.00000013; UTC=2017-04-18 18:42:12 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17041818-0025-0000-0000-00004377D5EF Message-Id: <20170418184207.GA3956@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-04-18_15:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1704180147 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 17, 2017 at 05:18:25PM -0700, Josh Triplett wrote: > 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. Right you are! Fixed, thank you! Thanx, Paul