From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752830AbaHMOM1 (ORCPT ); Wed, 13 Aug 2014 10:12:27 -0400 Received: from e38.co.us.ibm.com ([32.97.110.159]:59745 "EHLO e38.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751145AbaHMOMZ (ORCPT ); Wed, 13 Aug 2014 10:12:25 -0400 Date: Wed, 13 Aug 2014 07:12:17 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: 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, rostedt@goodmis.org, 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: <20140813141217.GU4752@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140813134025.GN9918@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: 14081314-1344-0000-0000-0000036F981F Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 13, 2014 at 03:40:25PM +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. Does your patch even work? Looks like it should, and yes, the idle loop seems quite a bit simpler than it was a few years ago, but we really don't need some strange thing that leaves a CPU idle but not visible as such to RCU. I have already said that I will be happy to rip out the wakeup code when it is no longer needed, and I agree that it would be way better if not needed. But I won't base a patch on hypotheticals. You have already drawn way too much water from -that- well over the past years! ;-) Thanx, Paul