From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752141AbdANJQK (ORCPT ); Sat, 14 Jan 2017 04:16:10 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:49591 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751685AbdANJNd (ORCPT ); Sat, 14 Jan 2017 04:13:33 -0500 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, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 13/20] rcu: Don't wake rcuc/X kthreads on NOCB CPUs Date: Sat, 14 Jan 2017 01:13:14 -0800 X-Mailer: git-send-email 2.5.2 In-Reply-To: <20170114091255.GA20854@linux.vnet.ibm.com> References: <20170114091255.GA20854@linux.vnet.ibm.com> X-TM-AS-GCONF: 00 X-Content-Scanned: Fidelis XPS MAILER x-cbid: 17011409-0024-0000-0000-000015B2C8C2 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006431; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000199; SDB=6.00807466; UDB=6.00393077; IPR=6.00584804; BA=6.00005055; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00013919; XFM=3.00000011; UTC=2017-01-14 09:13:30 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17011409-0025-0000-0000-000047F130E9 Message-Id: <1484385201-22227-13-git-send-email-paulmck@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-01-14_01:,, 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=8.0.1-1612050000 definitions=main-1701140136 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Chris Friesen notice that rcuc/X kthreads were consuming CPU even on NOCB CPUs. This makes no sense because the only purpose or these kthreads is to invoke normal (non-offloaded) callbacks, of which there will never be any on NOCB CPUs. This problem was due to a bug in cpu_has_callbacks_ready_to_invoke(), which should have been checking ->nxttail[RCU_NEXT_TAIL] for NULL, but which was instead (incorrectly) checking ->nxttail[RCU_DONE_TAIL]. Because ->nxttail[RCU_DONE_TAIL] is never NULL, the only effect is to cause the rcuc/X kthread to execute when it should not do so. This commit therefore checks ->nxttail[RCU_NEXT_TAIL], which is NULL for NOCB CPUs. Reported-by: Chris Friesen Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index ed5a17aca281..cac80e08b353 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -611,7 +611,7 @@ static int cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp) { return &rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL] && - rdp->nxttail[RCU_DONE_TAIL] != NULL; + rdp->nxttail[RCU_NEXT_TAIL] != NULL; } /* -- 2.5.2