From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752426AbaG0XiZ (ORCPT ); Sun, 27 Jul 2014 19:38:25 -0400 Received: from mail-yh0-f43.google.com ([209.85.213.43]:40335 "EHLO mail-yh0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750871AbaG0XiY (ORCPT ); Sun, 27 Jul 2014 19:38:24 -0400 From: Pranith Kumar To: "Paul E. McKenney" , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , linux-kernel@vger.kernel.org (open list:READ-COPY UPDATE...) Subject: [PATCH 1/1] rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads Date: Sun, 27 Jul 2014 19:37:29 -0400 Message-Id: <1406504249-15176-1-git-send-email-bobby.prani@gmail.com> X-Mailer: git-send-email 1.9.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The rcu_gp_kthread_wake() function checks for three conditions before waking up grace period kthreads: * Is the thread we are trying to wake up the current thread? * Are the gp_flags zero? (all threads wait on non-zero gp_flags condition) * Is there no thread created for this flavour, hence nothing to wake up? If any one of these condition is true, we do not call wake_up(). It was found that there are quite a few avoidable wake ups both during idle time and under stress induced by rcutorture. Idle: Total:66000, unnecessary:66000, case1:61827, case2:66000, case3:0 Total:68000, unnecessary:68000, case1:63696, case2:68000, case3:0 rcutorture: Total:254000, unnecessary:254000, case1:199913, case2:254000, case3:0 Total:256000, unnecessary:256000, case1:201784, case2:256000, case3:0 Here case{1-3} are the cases listed above. We can avoid these wake ups by using rcu_gp_kthread_wake() to conditionally wake up the grace period kthreads. Hence this commit tries to avoid calling wake_up() whenever we can by using rcu_gp_kthread_wake() function. Signed-off-by: Pranith Kumar --- kernel/rcu/tree.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index b63517c..36911ee 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -1938,7 +1938,10 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags) { WARN_ON_ONCE(!rcu_gp_in_progress(rsp)); raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags); - wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */ + /* ->gp_flags is properly protected by locks, so a memory barrier + * is not necessary here + */ + rcu_gp_kthread_wake(rsp); } /* @@ -2516,7 +2519,10 @@ static void force_quiescent_state(struct rcu_state *rsp) ACCESS_ONCE(rsp->gp_flags) = ACCESS_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS; raw_spin_unlock_irqrestore(&rnp_old->lock, flags); - wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */ + /* ->gp_flags is properly protected by locks, so a memory barrier + * is not necessary here + */ + rcu_gp_kthread_wake(rsp); } /* -- 1.9.1