From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752265AbeEKQZc (ORCPT ); Fri, 11 May 2018 12:25:32 -0400 Received: from mail.kernel.org ([198.145.29.99]:44486 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751398AbeEKQZb (ORCPT ); Fri, 11 May 2018 12:25:31 -0400 Date: Fri, 11 May 2018 12:25:28 -0400 From: Steven Rostedt To: "Paul E. McKenney" Cc: Byungchul Park , jiangshanlai@gmail.com, josh@joshtriplett.org, mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org, kernel-team@lge.com, peterz@infradead.org Subject: Re: [PATCH] rcu: Report a quiescent state when it's exactly in the state Message-ID: <20180511122528.2a398d24@gandalf.local.home> In-Reply-To: <20180511122321.722a12cc@gandalf.local.home> References: <1526027434-21237-1-git-send-email-byungchul.park@lge.com> <3af4cec0-4019-e3ac-77f9-8631252fb6da@lge.com> <20180511161746.GX26088@linux.vnet.ibm.com> <20180511122321.722a12cc@gandalf.local.home> X-Mailer: Claws Mail 3.16.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 11 May 2018 12:23:21 -0400 Steven Rostedt wrote: > On Fri, 11 May 2018 09:17:46 -0700 > "Paul E. McKenney" wrote: > > > > >index ee8cf5fc..7432261 100644 > > > >--- a/include/linux/rcupdate.h > > > >+++ b/include/linux/rcupdate.h > > > >@@ -195,8 +195,8 @@ static inline void exit_tasks_rcu_finish(void) { } > > > > */ > > > > #define cond_resched_tasks_rcu_qs() \ > > > > do { \ > > > >- if (!cond_resched()) \ > > > >- rcu_note_voluntary_context_switch_lite(current); \ > > > >+ rcu_note_voluntary_context_switch_lite(current); \ > > > >+ cond_resched(); \ > > > > Ah, good point. > > > > Peter, I have to ask... Why is "cond_resched()" considered a preemption > > while "schedule()" is not? > > I would argue that cond_resched() not be considered a preemption. > Although, it may be called a "preemption point". A place that can be > preempted, but may not be. Maybe that's the answer. schedule() will > always schedule (even though it may pick the same task to run, but > not guaranteed to), where as, cond_resched() will only schedule if the > conditions are right. And maybe that's not really a "voluntary > schedule", although I think that can be argued against. > I would also say that one should never call schedule() directly without changing its state to something other than TASK_RUNNING. Hence, calling schedule directly is saying you are ready to sleep. But that is not the case with cond_resched() which should always be called with the state as TASK_RUNNING. -- Steve