Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S938503AbdDSQ5T (ORCPT ); Wed, 19 Apr 2017 12:57:19 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:48633 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S938210AbdDSQqu (ORCPT ); Wed, 19 Apr 2017 12:46:50 -0400 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, 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, "Paul E. McKenney" Subject: [PATCH v3 tip/core/rcu 04/11] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick Date: Wed, 19 Apr 2017 09:46:30 -0700 X-Mailer: git-send-email 2.5.2 In-Reply-To: <20170419164553.GA10874@linux.vnet.ibm.com> References: <20170419164553.GA10874@linux.vnet.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 17041916-0040-0000-0000-0000031CB2E8 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006939; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000208; SDB=6.00849679; UDB=6.00419581; IPR=6.00628312; BA=6.00005304; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015095; XFM=3.00000013; UTC=2017-04-19 16:46:42 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17041916-0041-0000-0000-00000710C5BB Message-Id: <1492620397-12313-4-git-send-email-paulmck@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-04-19_14:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=1 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1703280000 definitions=main-1704190139 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1429 Lines: 37 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..c549618c72f0 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. Take the default if unsure. -- 2.5.2