From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1425400AbdEYWTp (ORCPT ); Thu, 25 May 2017 18:19:45 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:51739 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S942580AbdEYWAM (ORCPT ); Thu, 25 May 2017 18:00:12 -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 tip/core/rcu 06/88] rcu: Prevent rcu_barrier() from starting needless grace periods Date: Thu, 25 May 2017 14:58:39 -0700 X-Mailer: git-send-email 2.5.2 In-Reply-To: <20170525215934.GA11578@linux.vnet.ibm.com> References: <20170525215934.GA11578@linux.vnet.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 17052522-0044-0000-0000-000003398BB4 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007117; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000212; SDB=6.00865574; UDB=6.00429833; IPR=6.00645396; BA=6.00005375; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00015583; XFM=3.00000015; UTC=2017-05-25 22:00:08 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17052522-0045-0000-0000-0000076799C7 Message-Id: <1495749601-21574-6-git-send-email-paulmck@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-05-25_17:,, 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-1705250401 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Currently rcu_barrier() uses call_rcu() to enqueue new callbacks on each CPU with a non-empty callback list. This works, but means that rcu_barrier() forces grace periods that are not otherwise needed. The key point is that rcu_barrier() never needs to wait for a grace period, but instead only for all pre-existing callbacks to be invoked. This means that rcu_barrier()'s new callbacks should be placed in the callback-list segment containing the last pre-existing callback. This commit makes this change using the new rcu_segcblist_entrain() function. Signed-off-by: Paul E. McKenney --- include/trace/events/rcu.h | 1 + kernel/rcu/tree.c | 10 ++++++++-- 2 files changed, 9 insertions(+), 2 deletions(-) diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h index e3facb356838..91dc089d65b7 100644 --- a/include/trace/events/rcu.h +++ b/include/trace/events/rcu.h @@ -742,6 +742,7 @@ TRACE_EVENT(rcu_torture_read, * "OnlineQ": _rcu_barrier() found online CPU with callbacks. * "OnlineNQ": _rcu_barrier() found online CPU, no callbacks. * "IRQ": An rcu_barrier_callback() callback posted on remote CPU. + * "IRQNQ": An rcu_barrier_callback() callback found no callbacks. * "CB": An rcu_barrier_callback() invoked a callback, not the last. * "LastCB": An rcu_barrier_callback() invoked the last callback. * "Inc2": _rcu_barrier() piggyback check counter incremented. diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index e354e475e645..657056c3e0cd 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3578,8 +3578,14 @@ static void rcu_barrier_func(void *type) struct rcu_data *rdp = raw_cpu_ptr(rsp->rda); _rcu_barrier_trace(rsp, "IRQ", -1, rsp->barrier_sequence); - atomic_inc(&rsp->barrier_cpu_count); - rsp->call(&rdp->barrier_head, rcu_barrier_callback); + rdp->barrier_head.func = rcu_barrier_callback; + debug_rcu_head_queue(&rdp->barrier_head); + if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head, 0)) { + atomic_inc(&rsp->barrier_cpu_count); + } else { + debug_rcu_head_unqueue(&rdp->barrier_head); + _rcu_barrier_trace(rsp, "IRQNQ", -1, rsp->barrier_sequence); + } } /* -- 2.5.2