From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755143AbaHFMKK (ORCPT ); Wed, 6 Aug 2014 08:10:10 -0400 Received: from e32.co.us.ibm.com ([32.97.110.150]:56954 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752588AbaHFMKI (ORCPT ); Wed, 6 Aug 2014 08:10:08 -0400 Date: Wed, 6 Aug 2014 05:09:59 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Steven Rostedt , 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, 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: <20140806120958.GZ8101@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <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> <20140806012139.GY8101@linux.vnet.ibm.com> <20140806084708.GR9918@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140806084708.GR9918@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14080612-0928-0000-0000-000003EAC10D Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 06, 2014 at 10:47:08AM +0200, Peter Zijlstra wrote: > On Tue, Aug 05, 2014 at 06:21:39PM -0700, Paul E. McKenney wrote: > > > 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. > > Or you could shoot all CPUs with resched_cpu() which would have them > cycle through schedule() even if there's nothing but the idle thread to > run. That guarantees they'll go to sleep again in a !trampoline. Good point, that would be an easier way to handle the idle threads than messing with rcu_tasks_kthread()'s affinity. Thank you! > But I still very much hate the polling stuff... > > Can't we abuse the preempt notifiers? Say we make it possible to install > preemption notifiers cross-task, then the task-rcu can install a > preempt-out notifier which completes the rcu-task wait. > > After all, since we tagged it it was !running, and being scheduled out > means it ran (once) and therefore isn't on a trampoline anymore. Maybe I am being overly paranoid, but couldn't the task be preempted in a trampoline, be resumed, execute one instruction (still in the tramopoline) and be preempted again? > And the tick, which checks to see if the task got to userspace can do > the same, remove the notifier and then complete. My main concern with this sort of approach is that I have to deal with full-up concurrency (200 CPUs all complete tasks concurrently, for example), which would make for a much larger and more complex patch. Now, I do admit that it is quite possible that I will end up there anyway, for example, if more people start using RCU-tasks, but I see no need to hurry this process. ;-) Thanx, Paul