From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754534AbbAGRVK (ORCPT ); Wed, 7 Jan 2015 12:21:10 -0500 Received: from e36.co.us.ibm.com ([32.97.110.154]:40823 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754230AbbAGRVI (ORCPT ); Wed, 7 Jan 2015 12:21:08 -0500 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, 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, Alexander Gordeev , "Paul E. McKenney" Subject: [PATCH tip/core/rcu 02/10] rcu: Remove redundant rcu_is_cpu_rrupt_from_idle() from tiny RCU Date: Wed, 7 Jan 2015 09:20:49 -0800 Message-Id: <1420651257-553-2-git-send-email-paulmck@linux.vnet.ibm.com> X-Mailer: git-send-email 1.8.1.5 In-Reply-To: <1420651257-553-1-git-send-email-paulmck@linux.vnet.ibm.com> References: <20150107172030.GA31571@linux.vnet.ibm.com> <1420651257-553-1-git-send-email-paulmck@linux.vnet.ibm.com> X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15010717-0021-0000-0000-00000793775D Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Alexander Gordeev Let's start assuming that something in the idle loop posts a callback, and scheduling-clock interrupt occurs: 1. The system is idle and stays that way, no runnable tasks. 2. Scheduling-clock interrupt occurs, rcu_check_callbacks() is called as result, which in turn calls rcu_is_cpu_rrupt_from_idle(). 3. rcu_is_cpu_rrupt_from_idle() reports the CPU was interrupted from idle, which results in rcu_sched_qs() call, which does a raise_softirq(RCU_SOFTIRQ). 4. Upon return from interrupt, rcu_irq_exit() is invoked, which calls rcu_idle_enter_common(), which in turn calls rcu_sched_qs() again, which does another raise_softirq(RCU_SOFTIRQ). 5. The softirq happens shortly and invokes rcu_process_callbacks(), which invokes __rcu_process_callbacks(). 6. So now callbacks can be invoked. At least they can be if ->donetail has been updated. Which it will have been because rcu_sched_qs() invokes rcu_qsctr_help(). In the described scenario rcu_sched_qs() and raise_softirq(RCU_SOFTIRQ) get called twice in steps 3 and 4. This redundancy could be eliminated by removing rcu_is_cpu_rrupt_from_idle() function. Signed-off-by: Alexander Gordeev Cc: Paul E. McKenney Signed-off-by: Paul E. McKenney --- kernel/rcu/tiny.c | 12 +----------- 1 file changed, 1 insertion(+), 11 deletions(-) diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c index 0db5649f8817..805b6d59c854 100644 --- a/kernel/rcu/tiny.c +++ b/kernel/rcu/tiny.c @@ -186,16 +186,6 @@ EXPORT_SYMBOL(__rcu_is_watching); #endif /* defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) */ /* - * Test whether the current CPU was interrupted from idle. Nested - * interrupts don't count, we must be running at the first interrupt - * level. - */ -static int rcu_is_cpu_rrupt_from_idle(void) -{ - return rcu_dynticks_nesting <= 1; -} - -/* * Helper function for rcu_sched_qs() and rcu_bh_qs(). * Also irqs are disabled to avoid confusion due to interrupt handlers * invoking call_rcu(). @@ -250,7 +240,7 @@ void rcu_bh_qs(void) void rcu_check_callbacks(int user) { RCU_TRACE(check_cpu_stalls()); - if (user || rcu_is_cpu_rrupt_from_idle()) + if (user) rcu_sched_qs(); else if (!in_softirq()) rcu_bh_qs(); -- 1.8.1.5