From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751915AbaGXEEI (ORCPT ); Thu, 24 Jul 2014 00:04:08 -0400 Received: from mail-lb0-f179.google.com ([209.85.217.179]:33253 "EHLO mail-lb0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751177AbaGXEEG (ORCPT ); Thu, 24 Jul 2014 00:04:06 -0400 MIME-Version: 1.0 In-Reply-To: <20140724034315.GJ11241@linux.vnet.ibm.com> References: <1406092194-13004-1-git-send-email-bobby.prani@gmail.com> <1406092194-13004-12-git-send-email-bobby.prani@gmail.com> <20140723122608.GL11241@linux.vnet.ibm.com> <20140724034315.GJ11241@linux.vnet.ibm.com> From: Pranith Kumar Date: Thu, 24 Jul 2014 00:03:34 -0400 Message-ID: Subject: Re: [PATCH 11/16] rcu: Check for spurious wakeup using return value To: Paul McKenney Cc: Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , "open list:READ-COPY UPDATE..." Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 23, 2014 at 11:43 PM, Paul E. McKenney wrote: > On Wed, Jul 23, 2014 at 10:36:19PM -0400, Pranith Kumar wrote: >> On Wed, Jul 23, 2014 at 8:26 AM, Paul E. McKenney >> wrote: >> > On Wed, Jul 23, 2014 at 01:09:48AM -0400, Pranith Kumar wrote: >> >> When the gp_kthread wakes up from the wait event, it returns 0 if the wake up is >> >> due to the condition having been met. This commit checks this return value >> >> for a spurious wake up before calling rcu_gp_init(). >> >> >> >> Signed-off-by: Pranith Kumar >> > >> > How does this added check help? I don't see that it does. If the flag >> > is set, we want to wake up. If we get a spurious wakeup, but then the >> > flag gets set before we actually wake up, we still want to wake up. >> >> So I took a look at the docs again, and using the return value is the >> recommended way to check for spurious wakeups. >> >> The condition in wait_event_interruptible() is checked when the task >> is woken up (either due to stray signals or explicitly) and it returns >> true if condition evaluates to true. this should be returns '0' if the condition evaluates to true. >> >> In the current scenario, if we get a spurious wakeup, we take the >> costly path of checking this condition again (with a barrier and lock) >> before going back to wait. >> >> The scenario of getting an actual wakeup after getting a spurious >> wakeup exists even today, this is the window after detecting a >> spurious wakeup and before going back to wait. I am not sure if using >> the return value enlarges that window as we are going back to sleep >> immediately. >> >> Thoughts? > > If the flag is set, why should we care whether or not the wakeup was > spurious? If the flag is not set, why should we care whether or not > wait_event_interruptible() thought that the wakeup was not spurious? > A correction about the return value above: return will be 0 if the condition is true, in this case if the flag is set. If the flag is set, ret will be 0 and we will go ahead with rcu_gp_init(). (no change wrt current behavior) If the flag is not set, currently we go ahead and call rcu_gp_init() from where we check if the flag is set (after a lock+barrier) and return. If we care about what wait_event_interruptible() returns, we can go back and wait for an actual wakeup much earlier without the additional overhead of calling rcu_gp_init(). -- Pranith