Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760140AbaJaOq6 (ORCPT ); Fri, 31 Oct 2014 10:46:58 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55390 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756578AbaJaOq5 (ORCPT ); Fri, 31 Oct 2014 10:46:57 -0400 Date: Fri, 31 Oct 2014 14:55:05 +0000 From: Alexander Gordeev To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org Subject: [PATCH v2] rcu: Remove redundant rcu_is_cpu_rrupt_from_idle() from tiny RCU Message-ID: <20141031145504.GC18126@agordeev.usersys.redhat.com> References: <20141029143439.GN5718@linux.vnet.ibm.com> <20141029151622.GD30198@agordeev.usersys.redhat.com> <20141029154615.GR5718@linux.vnet.ibm.com> <20141029210916.GB13273@agordeev.usersys.redhat.com> <20141030000524.GG5718@linux.vnet.ibm.com> <20141030175435.GC13273@agordeev.usersys.redhat.com> <20141030204318.GR5718@linux.vnet.ibm.com> <20141030215433.GA18126@agordeev.usersys.redhat.com> <20141031034559.GT5718@linux.vnet.ibm.com> <20141031145326.GB18126@agordeev.usersys.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141031145326.GB18126@agordeev.usersys.redhat.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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 c0623fc..0bfb3b9 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 cpu, 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(); -- 1.8.3.1 -- Regards, Alexander Gordeev agordeev@redhat.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/