From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH [RT] 08/14] add a loop counter based timeout mechanism Date: Fri, 22 Feb 2008 11:43:41 -0800 Message-ID: <20080222194341.GE11213@linux.vnet.ibm.com> References: <20080221152504.4804.8724.stgit@novell1.haskins.net> <20080221152707.4804.59177.stgit@novell1.haskins.net> <200802211741.10299.ak@suse.de> <20080222190814.GD11213@linux.vnet.ibm.com> <9810cff90802221119j23818e74g2721512a693a0a01@mail.gmail.com> <9810cff90802221121s216f69f4k4a5f39eaaf11dd7f@mail.gmail.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Andi Kleen , Gregory Haskins , mingo@elte.hu, a.p.zijlstra@chello.nl, tglx@linutronix.de, rostedt@goodmis.org, linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, kevin@hilman.org, cminyard@mvista.com, dsingleton@mvista.com, dwalker@mvista.com, npiggin@suse.de, dsaxena@plexity.net, gregkh@suse.de, sdietrich@novell.com, pmorreale@novell.com, mkohari@novell.com To: "Bill Huey (hui)" Return-path: Received: from e34.co.us.ibm.com ([32.97.110.152]:50206 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753353AbYBVTn7 (ORCPT ); Fri, 22 Feb 2008 14:43:59 -0500 Content-Disposition: inline In-Reply-To: <9810cff90802221121s216f69f4k4a5f39eaaf11dd7f@mail.gmail.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On Fri, Feb 22, 2008 at 11:21:14AM -0800, Bill Huey (hui) wrote: > On Fri, Feb 22, 2008 at 11:19 AM, Bill Huey (hui) wrote: > > Yeah, I'm not very keen on having a constant there without some > > contention instrumentation to see how long the spins are. It would be > > better to just let it run until either task->on_cpu is off or checking > > if the "current" in no longer matches the mutex owner for the runqueue > > in question. At that point, you know the thread isn't running. > > Spinning on something like that is just a waste of time. It's for that > > reason that doing in the spin outside of a preempt critical section > > isn't really needed > > Excuse me, I meant to say "...isn't a problem". The fixed-time spins are very useful in cases where the critical section is almost always very short but can sometimes be very long. In such cases, you would want to spin until either ownership changes or it is apparent that the current critical-section instance will be long. I believe that there are locks in the Linux kernel that have this "mostly short but sometimes long" hold-time property. Thanx, Paul