From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753353AbaHMU4b (ORCPT ); Wed, 13 Aug 2014 16:56:31 -0400 Received: from e35.co.us.ibm.com ([32.97.110.153]:57109 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751529AbaHMU4a (ORCPT ); Wed, 13 Aug 2014 16:56:30 -0400 Date: Wed, 13 Aug 2014 13:56:23 -0700 From: "Paul E. McKenney" To: Steven Rostedt Cc: Peter Zijlstra , 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, oleg@redhat.com, bobby.prani@gmail.com Subject: Re: [PATCH v5 tip/core/rcu 15/16] rcu: Make RCU-tasks wait for idle tasks Message-ID: <20140813205623.GA7967@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140811224840.GA25594@linux.vnet.ibm.com> <1407797345-28227-1-git-send-email-paulmck@linux.vnet.ibm.com> <1407797345-28227-15-git-send-email-paulmck@linux.vnet.ibm.com> <20140813081215.GB9918@twins.programming.kicks-ass.net> <20140813124818.GQ4752@linux.vnet.ibm.com> <20140813134025.GN9918@twins.programming.kicks-ass.net> <20140813095132.56d288f2@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140813095132.56d288f2@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: 14081320-6688-0000-0000-000003FDC8E8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 13, 2014 at 09:51:32AM -0400, Steven Rostedt wrote: > On Wed, 13 Aug 2014 15:40:25 +0200 > Peter Zijlstra wrote: > > > On Wed, Aug 13, 2014 at 05:48:18AM -0700, Paul E. McKenney wrote: > > > On Wed, Aug 13, 2014 at 10:12:15AM +0200, Peter Zijlstra wrote: > > > > On Mon, Aug 11, 2014 at 03:49:04PM -0700, Paul E. McKenney wrote: > > > > > From: "Paul E. McKenney" > > > > > > > > > > Because idle-task code may need to be patched, RCU-tasks need to wait > > > > > for idle tasks to schedule. This commit therefore detects this case > > > > > via context switch. Block CPU hotplug during this time to avoid sending > > > > > IPIs to offline CPUs. > > > > > > > > > > Note that checking for changes in the dyntick-idle counters is tempting, > > > > > but wrong. The reason that it is wrong is that a interrupt or NMI can > > > > > increment these counters without necessarily allowing the idle tasks to > > > > > make any forward progress. > > > > > > > > I'm going to NAK this.. with that rcu_idle patch I send there's > > > > typically only a single idle function thats out of bounds and if its > > > > more it can be made that with a bit of tlc to the cpuidle driver in > > > > question. > > > > > > > > This needs _FAR_ more justification than a maybe and a want. > > > > > > Peter, your patch might be a good start, but I didn't see any reaction > > > from Steven or Masami and it did only x86. > > > > That's not an excuse for doing horrible things. And inventing new infra > > that needs to wake all CPUs is horrible. > > I still need to look at the patches, but if this is just for the idle > case, then we don't need it. The idle case can be solved with a simple > sched_on_each_cpu(). I need a way to solve waiting for processes to > finish from a preemption point. > > That's all I want, and if we can remove the "idle" case and document it > well that it's not covered and a sched_on_each_cpu() may be needed, > then I'm fine with that. > > sched_on_each_cpu(dummy_op); > call_rcu_tasks(free_tramp); > > Would that work? If you are taking that approach, I can of course drop my commit dealing with idle tasks. Should the rcu_idle_enter() and rcu_idle_exit() calls avoid cover any functions needing trampolines, it would be easy to pull them back in -- especially given that the RCU dyntick-idle information would call out the quiescent states appropriately. So unless you tell me otherwise, Steven, I will drop the idle-detection commit in favor of your sched_on_each_cpu() approach. Thanx, Paul