From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932746Ab3IYBax (ORCPT ); Tue, 24 Sep 2013 21:30:53 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:53562 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756958Ab3IYB3b (ORCPT ); Tue, 24 Sep 2013 21:29:31 -0400 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 08/11] rcu: Micro-optimize rcu_cpu_has_callbacks() Date: Tue, 24 Sep 2013 18:29:18 -0700 Message-Id: <1380072561-31134-8-git-send-email-paulmck@linux.vnet.ibm.com> X-Mailer: git-send-email 1.8.1.5 In-Reply-To: <1380072561-31134-1-git-send-email-paulmck@linux.vnet.ibm.com> References: <20130925012750.GA30601@linux.vnet.ibm.com> <1380072561-31134-1-git-send-email-paulmck@linux.vnet.ibm.com> X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13092501-7182-0000-0000-000008839849 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Paul E. McKenney" The for_each_rcu_flavor() loop unconditionally scans all flavors, even when the first flavor might have some non-lazy callbacks. Once the loop has seen a non-lazy callback, further passes through the loop cannot change the state. This is not a huge problem, given that there can be at most three RCU flavors (RCU-bh, RCU-preempt, and RCU-sched), but this code is on the path to idle, so speeding it up even a small amount would have some benefit. This commit therefore does two things: 1. Rearranges the order of the list of RCU flavors in order to place the most active flavor first in the list. The most active RCU flavor is RCU-preempt, or, if there is no RCU-preempt, RCU-sched. 2. Reworks the for_each_rcu_flavor() to exit early when the first non-lazy callback is seen, or, in the case where the caller does not care about non-lazy callbacks (RCU_FAST_NO_HZ=n), when the first callback is seen. Reported-by: Chen Gang Signed-off-by: Paul E. McKenney --- kernel/rcutree.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/kernel/rcutree.c b/kernel/rcutree.c index e6f2e8f..49464ad 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -2727,10 +2727,13 @@ static int rcu_cpu_has_callbacks(int cpu, bool *all_lazy) for_each_rcu_flavor(rsp) { rdp = per_cpu_ptr(rsp->rda, cpu); - if (rdp->qlen != rdp->qlen_lazy) + if (!rdp->nxtlist) + continue; + hc = true; + if (rdp->qlen != rdp->qlen_lazy || !all_lazy) { al = false; - if (rdp->nxtlist) - hc = true; + break; + } } if (all_lazy) *all_lazy = al; @@ -3297,8 +3300,8 @@ void __init rcu_init(void) rcu_bootup_announce(); rcu_init_geometry(); - rcu_init_one(&rcu_sched_state, &rcu_sched_data); rcu_init_one(&rcu_bh_state, &rcu_bh_data); + rcu_init_one(&rcu_sched_state, &rcu_sched_data); __rcu_init_preempt(); open_softirq(RCU_SOFTIRQ, rcu_process_callbacks); -- 1.8.1.5