From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755015AbdDLQnS (ORCPT ); Wed, 12 Apr 2017 12:43:18 -0400 Received: from mail.kernel.org ([198.145.29.136]:32970 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754116AbdDLQnP (ORCPT ); Wed, 12 Apr 2017 12:43:15 -0400 Date: Wed, 12 Apr 2017 12:43:09 -0400 From: Steven Rostedt To: Peter Zijlstra Cc: LKML , Ingo Molnar , "Paul E. McKenney" Subject: Re: [RFC][PATCH] sched: Have do_idle() call __schedule() without enabling preemption Message-ID: <20170412124309.71cb3348@gandalf.local.home> In-Reply-To: <20170412162157.ukmhksytgrc2qx2e@hirez.programming.kicks-ass.net> References: <20170411224413.72d9344a@grimm.local.home> <20170412104124.4639c8ff@gandalf.local.home> <20170412162157.ukmhksytgrc2qx2e@hirez.programming.kicks-ass.net> X-Mailer: Claws Mail 3.14.0 (GTK+ 2.24.31; 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 Wed, 12 Apr 2017 18:21:57 +0200 Peter Zijlstra wrote: > On Wed, Apr 12, 2017 at 10:41:24AM -0400, Steven Rostedt wrote: > > [ tl;dr; version ] > > > > Peter, In order to have synchronize_rcu_tasks() work, the idle task can > > never be preempted. There's a very small window in > > schedule_preempt_disable() that enables preemption, and when this > > happens, it breaks synchronize_rcu_tasks() (see above email for > > details). > > > > Is there any reason to enable preemption, or can we simply have idle > > call into schedule without ever allowing it to be preempted, as in my > > patch? > > Dunno,.. this changelog should convince me, not make me do the work :-) I didn't want you to do any work. I was wondering if you knew of any reason. But looking into the history of the do_idle() function and schedule_preempt_disable(), I don't see any reason. I'll post another patch with a better change log. > > > Note, it is almost good enough to just change > > schedule_preempt_disable() to do the exact same thing, but there's one > > instance in kernel/locking/mutex.c that calls it in a non running state. > > The point being that that must not happen because of > sched_submit_work(), which, for idle, should not matter. Right, and also for all the other cases that call schedule_preempt_disable() which includes init/main.c and another location in mutex.c that sets the task state to TASK_RUNNING just before calling it. But the one case where it can be something other than TASK_RUNNING, then we need to keep the current method. OK, I think it is fine for the idle task to never enable preemption and I'll post a better change log patch. Thanks! -- Steve