From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754802AbaHMW4b (ORCPT ); Wed, 13 Aug 2014 18:56:31 -0400 Received: from e36.co.us.ibm.com ([32.97.110.154]:36409 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753014AbaHMW43 (ORCPT ); Wed, 13 Aug 2014 18:56:29 -0400 Date: Wed, 13 Aug 2014 15:56:23 -0700 From: "Paul E. McKenney" To: Pranith Kumar Cc: Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , "open list:READ-COPY UPDATE..." Subject: Re: [PATCH v2] rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads Message-ID: <20140813225623.GE4752@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1407863267-27559-1-git-send-email-bobby.prani@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1407863267-27559-1-git-send-email-bobby.prani@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14081322-3532-0000-0000-000003D41CF3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 12, 2014 at 01:07:47PM -0400, Pranith Kumar wrote: > Updated changelog regarding the removed comment about the implied barrier > provided by wake_up() logic. > > -- > Pranith Queued for 3.18, thank you! Thanx, Paul > 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. > > There is a comment about an implied barrier supplied by the wake_up() logic. > This barrier is necessary for the awakened thread to see the updated ->gp_flags. > This flag is always being updated with the root node lock held. Also, the > awakened thread tries to acquire the root node lock before reading ->gp_flags > because of which there is proper ordering. > > Hence this commit tries to avoid calling wake_up() whenever we can by using > rcu_gp_kthread_wake() function. > > Signed-off-by: Pranith Kumar > CC: Mathieu Desnoyers > --- > kernel/rcu/tree.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index b63517c..6b68d2d 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1938,7 +1938,7 @@ 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. */ > + rcu_gp_kthread_wake(rsp); > } > > /* > @@ -2516,7 +2516,7 @@ 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. */ > + rcu_gp_kthread_wake(rsp); > } > > /* > -- > 1.9.1 >