From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754091AbeENOzB (ORCPT ); Mon, 14 May 2018 10:55:01 -0400 Received: from mail.kernel.org ([198.145.29.99]:53504 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751498AbeENOy5 (ORCPT ); Mon, 14 May 2018 10:54:57 -0400 Date: Mon, 14 May 2018 10:54:54 -0400 From: Steven Rostedt To: "Joel Fernandes (Google)" Cc: linux-kernel@vger.kernel.org, "Paul E. McKenney" , Josh Triplett , Mathieu Desnoyers , Lai Jiangshan , byungchul.park@lge.com, kernel-team@android.com Subject: Re: [PATCH RFC 2/8] rcu: Clarify usage of cond_resched for tasks-RCU Message-ID: <20180514105454.45946ad3@gandalf.local.home> In-Reply-To: <20180514031541.67247-3-joel@joelfernandes.org> References: <20180514031541.67247-1-joel@joelfernandes.org> <20180514031541.67247-3-joel@joelfernandes.org> 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 Sun, 13 May 2018 20:15:35 -0700 "Joel Fernandes (Google)" wrote: > Recently we had a discussion about cond_resched unconditionally > recording a voluntary context switch [1]. > > Lets add a comment clarifying that how this API is to be used. > > [1] https://lkml.kernel.org/r/1526027434-21237-1-git-send-email-byungchul.park@lge.com > > Signed-off-by: Joel Fernandes (Google) > --- > include/linux/rcupdate.h | 11 ++++++++--- > 1 file changed, 8 insertions(+), 3 deletions(-) > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index 743226176350..a9881007ece6 100644 > --- a/include/linux/rcupdate.h > +++ b/include/linux/rcupdate.h > @@ -159,8 +159,12 @@ static inline void rcu_init_nohz(void) { } > } while (0) > > /* > - * Note a voluntary context switch for RCU-tasks benefit. This is a > - * macro rather than an inline function to avoid #include hell. > + * Note an attempt to perform a voluntary context switch for RCU-tasks benefit. > + * > + * This is called even in situations where a context switch didn't really > + * happen even though it was requested. The caller uses it to indicate > + * traversal of an RCU-tasks quiescent state. This is a macro rather than an > + * inline function to avoid #include hell. I don't know. I just don't like the wording. It sounds too much like it was written by someone that was confused for it being called when a context switch didn't occur ;-) What about something more like: /* * This is called to denote a RCU-task quiescent state. It is placed at * voluntary preemption points, as RCU-task critical sections may not * perform voluntary preemption or scheduling calls. It does not matter * if the task is scheduled out or not, just that a voluntary preemption * may be done. */ > */ > #ifdef CONFIG_TASKS_RCU > #define rcu_note_voluntary_context_switch_lite(t) \ > @@ -187,7 +191,8 @@ static inline void exit_tasks_rcu_finish(void) { } > #endif /* #else #ifdef CONFIG_TASKS_RCU */ > > /** > - * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU > + * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU. > + * The quiescent state report is made even if cond_resched() did nothing. Same thing here. * The quiescent state report does not depend on cond_resched() scheduling. -- Steve > * > * This macro resembles cond_resched(), except that it is defined to > * report potential quiescent states to RCU-tasks even if the cond_resched()