From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755501AbaHFBVs (ORCPT ); Tue, 5 Aug 2014 21:21:48 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:48450 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751085AbaHFBVr (ORCPT ); Tue, 5 Aug 2014 21:21:47 -0400 Date: Tue, 5 Aug 2014 18:21:39 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: Oleg Nesterov , linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, bobby.prani@gmail.com Subject: Re: [PATCH v3 tip/core/rcu 3/9] rcu: Add synchronous grace-period waiting for RCU-tasks Message-ID: <20140806012139.GY8101@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140731215445.GA21933@linux.vnet.ibm.com> <1406843709-23396-1-git-send-email-paulmck@linux.vnet.ibm.com> <1406843709-23396-3-git-send-email-paulmck@linux.vnet.ibm.com> <20140801150926.GA845@redhat.com> <20140801183251.GJ4784@linux.vnet.ibm.com> <20140801194417.GA27141@linux.vnet.ibm.com> <20140802144719.GA18018@redhat.com> <20140802225857.GC8101@linux.vnet.ibm.com> <20140805205711.7a52076c@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140805205711.7a52076c@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14080601-8236-0000-0000-000004623304 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Aug 05, 2014 at 08:57:11PM -0400, Steven Rostedt wrote: > On Sat, 2 Aug 2014 15:58:57 -0700 > "Paul E. McKenney" wrote: > > > On Sat, Aug 02, 2014 at 04:47:19PM +0200, Oleg Nesterov wrote: > > > On 08/01, Paul E. McKenney wrote: > > > > > > > > On Fri, Aug 01, 2014 at 11:32:51AM -0700, Paul E. McKenney wrote: > > > > > On Fri, Aug 01, 2014 at 05:09:26PM +0200, Oleg Nesterov wrote: > > > > > > On 07/31, Paul E. McKenney wrote: > > > > > > > > > > > > > > +void synchronize_rcu_tasks(void) > > > > > > > +{ > > > > > > > + /* Complain if the scheduler has not started. */ > > > > > > > + rcu_lockdep_assert(!rcu_scheduler_active, > > > > > > > + "synchronize_rcu_tasks called too soon"); > > > > > > > + > > > > > > > + /* Wait for the grace period. */ > > > > > > > + wait_rcu_gp(call_rcu_tasks); > > > > > > > +} > > > > > > > > > > > > Btw, what about CONFIG_PREEMPT=n ? > > > > > > > > > > > > I mean, can't synchronize_rcu_tasks() be synchronize_sched() in this > > > > > > case? > > > > > > > > > > Excellent point, indeed it can! > > > > > > > > > > And if I do it right, it will make CONFIG_TASKS_RCU=y safe for kernel > > > > > tinification. ;-) > > > > > > > > Unless, that is, we need to wait for trampolines in the idle loop... > > > > > > > > Sounds like a question for Steven. ;-) > > > > > > Sure, but the full blown synchronize_rcu_tasks() can't handle the idle threads > > > anyway. An idle thread can not be deactivated and for_each_process() can't see > > > it anyway. > > > > Indeed, if idle threads need to be tracked, their tracking will need to > > be at least partially special-cased. > > Yeah, idle threads can be affected by the trampolines. That is, we can > still hook a trampoline to some function in the idle loop. > > But we should be able to make the hardware call that puts the CPU to > sleep a quiescent state too. May need to be arch dependent. :-/ OK, my plan for this eventuality is to do the following: 1. Ignore the ->on_rq field, as idle tasks are always on a runqueue. 2. Watch the context-switch counter. 3. Ignore dyntick-idle state for idle tasks. 4. If there is no quiescent state from a given idle task after a few seconds, schedule rcu_tasks_kthread() on top of the offending CPU. Your idea is an interesting one, but does require another set of dyntick-idle-like functions and counters. Or moving the current rcu_idle_enter() and rcu_idle_exit() calls deeper into the idle loop. Not sure which is a better approach. Alternatively, we could just rely on #4 above, on the grounds that battery life should not be too badly degraded by the occasional RCU-tasks interference. Note that this is a different situation than NO_HZ_FULL in realtime environments, where the worst case causes trouble even if it happens very infrequently. Thanx, Paul