From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756667Ab0DACNZ (ORCPT ); Wed, 31 Mar 2010 22:13:25 -0400 Received: from e35.co.us.ibm.com ([32.97.110.153]:44626 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755834Ab0DACNY (ORCPT ); Wed, 31 Mar 2010 22:13:24 -0400 Message-ID: <4BB40140.20109@us.ibm.com> Date: Wed, 31 Mar 2010 19:13:20 -0700 From: Darren Hart User-Agent: Thunderbird 2.0.0.24 (X11/20100317) MIME-Version: 1.0 To: rostedt@goodmis.org CC: "lkml," , Peter Zijlstra , Gregory Haskins , Sven-Thorsten Dietrich , Peter Morreale , Thomas Gleixner , Ingo Molnar , Eric Dumazet , Chris Mason Subject: Re: RFC: Ideal Adaptive Spinning Conditions References: <4BB3D90C.3030108@us.ibm.com> <1270078689.19685.8040.camel@gandalf.stny.rr.com> In-Reply-To: <1270078689.19685.8040.camel@gandalf.stny.rr.com> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Steven Rostedt wrote: > On Wed, 2010-03-31 at 16:21 -0700, Darren Hart wrote: > >> o What type of lock hold times do we expect to benefit? > > 0 (that's a zero) :-p > > I haven't seen your patches but you are not doing a heuristic approach, > are you? That is, do not "spin" hoping the lock will suddenly become > free. I was against that for -rt and I would be against that for futex > too. I'm not sure what you're getting at here. Adaptive spinning is indeed hoping the lock will become free while you are spinning and checking it's owner... > >> o How much contention is a good match for adaptive spinning? >> - this is related to the number of threads to run in the test >> o How many spinners should be allowed? >> >> I can share the kernel patches if people are interested, but they are >> really early, and I'm not sure they are of much value until I better >> understand the conditions where this is expected to be useful. > > Again, I don't know how you implemented your adaptive spinners, but the > trick to it in -rt was that it would only spin while the owner of the > lock was actually running. If it was not running, it would sleep. No > point waiting for a sleeping task to release its lock. It does exactly this. > Is this what you did? Because, IIRC, this only benefited spinlocks > converted to mutexes. It did not help with semaphores, because > semaphores could be held for a long time. Thus, it was good for short > held locks, but hurt performance on long held locks. Trouble is, I'm still seeing performance penalties even on the shortest critical section possible (lock();unlock();) > If userspace is going to do this, I guess the blocked task would need to > go into kernel, and spin there (with preempt enabled) if the task is > still active and holding the lock. It is currently under preempt_disable() just like mutexes. I asked Peter why it was done that way for mutexes, but didn't really get an answer. He did point out that since we check need_resched() at every iteration that we won't run longer than our timeslice anyway, so it shouldn't be a problem. > Then the application would need to determine which to use. An adaptive > spinner for short held locks, and a normal futex for long held locks. Yes, this was intended to be an optional thing (and certainly not the default). -- Darren Hart IBM Linux Technology Center Real-Time Linux Team