From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754170Ab2DZRaF (ORCPT ); Thu, 26 Apr 2012 13:30:05 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:58820 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753632Ab2DZRaC (ORCPT ); Thu, 26 Apr 2012 13:30:02 -0400 Date: Thu, 26 Apr 2012 10:28:59 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, patches@linaro.org, "Paul E. McKenney" Subject: Re: [PATCH RFC tip/core/rcu 4/6] rcu: Clarify help text for RCU_BOOST_PRIO Message-ID: <20120426172859.GF2407@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20120423164159.GA13819@linux.vnet.ibm.com> <1335199347-13926-1-git-send-email-paulmck@linux.vnet.ibm.com> <1335199347-13926-4-git-send-email-paulmck@linux.vnet.ibm.com> <1335444391.13683.11.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1335444391.13683.11.camel@twins> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12042617-5806-0000-0000-000014A4A448 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 26, 2012 at 02:46:31PM +0200, Peter Zijlstra wrote: > On Mon, 2012-04-23 at 09:42 -0700, Paul E. McKenney wrote: > > + This option specifies the real-time priority to which long-term > > + preempted RCU readers are to be boosted. If you are working > > + with a real-time application that has one or more CPU-bound > > + threads running at a real-time priority level, > > Then your application is broken ;-) the kernel is known to mis-behave > under these circumstances since it doesn't get to run house-keeping > tasks. RCU is just one of these and elevating it doesn't make it work. As you say, CPU-bound RT tasks have a number of problems, and RCU is but one of them. That said, an RCU-induced memory-exhaustion system hang is an extremely unfriendly diagnostic message, and use of RCU priority boosting allows them a better debugging environment. > > you should set > > + RCU_BOOST_PRIO to a priority higher then the highest-priority > > + real-time CPU-bound thread. The default RCU_BOOST_PRIO value > > + of 1 is appropriate in the common case, which is real-time > > + applications that do not have any CPU-bound threads. > > Alternatively, 1 is the worst possible choice forcing people to consider > the issue. You say that as if forcing people to consider the issue was a bad thing. ;-) > > + Some real-time applications might not have a single real-time > > + thread that saturates a given CPU, but instead might have > > + multiple real-time threads that, taken together, fully utilize > > + that CPU. In this case, you should set RCU_BOOST_PRIO to > > + a priority higher than the lowest-priority thread that is > > + conspiring to prevent the CPU from running any non-real-time > > + tasks. For example, if one thread at priority 10 and another > > + thread at priority 5 are between themselves fully consuming > > + the CPU time on a given CPU, then RCU_BOOST_PRIO should be > > + set to priority 6 or higher. > > I'd call this misleading, who's to say that preempting the 5 would yield > enough time to complete the RCU work? Yep, hence the "or higher". > This all gets us back to the fun question of RCU delayed bandwidth > budgeting.. ideally every 'task' that does call_rcu() should donate some > of its budget towards the thread running the callback. There was an academic interested in that topic a few years ago, but I don't believe anything came of it. An interesting approach would be to do EDF scheduling on the callbacks themselves, but having a separate thread for each callback sounds like overkill. > Anyway, I'd argue both the old and new description are bonkers. Indeed, my goal was "less bonkers" rather than "not bonkers". A "not bonkers" description remains a long-term aspiration rather than a short-term goal for the moment. I can only hope that the timeframe is shorter than it was for RCU back in the early 1990s. ;-) Thanx, Paul