From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758069Ab0DACZ7 (ORCPT ); Wed, 31 Mar 2010 22:25:59 -0400 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.123]:57350 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757935Ab0DACZ5 (ORCPT ); Wed, 31 Mar 2010 22:25:57 -0400 X-Authority-Analysis: v=1.1 cv=pKw63bS+wPQ6BxbjPFuXko/1Hb8f7lpGL8zfzHecwYY= c=1 sm=0 a=5_LdwiuLWHQA:10 a=7U3hwN5JcxgA:10 a=Q9fys5e9bTEA:10 a=gMqfjgEr1zLu/65IO0LwxA==:17 a=63SHTuVW8WvA3R2mAQsA:9 a=NdVsBFc6du9G6Am1Q1cA:7 a=k9FVD6jhqoN3OoOvAIu4d-0Omg4A:4 a=PUjeQqilurYA:10 a=IePpGRbsbpkco2dL:21 a=8XrVUr-FaGlq3Xsk:21 a=gMqfjgEr1zLu/65IO0LwxA==:117 X-Cloudmark-Score: 0 X-Originating-IP: 74.67.89.75 Subject: Re: RFC: Ideal Adaptive Spinning Conditions From: Steven Rostedt Reply-To: rostedt@goodmis.org To: Darren Hart Cc: "lkml," , Peter Zijlstra , Gregory Haskins , Sven-Thorsten Dietrich , Peter Morreale , Thomas Gleixner , Ingo Molnar , Eric Dumazet , Chris Mason In-Reply-To: <4BB40140.20109@us.ibm.com> References: <4BB3D90C.3030108@us.ibm.com> <1270078689.19685.8040.camel@gandalf.stny.rr.com> <4BB40140.20109@us.ibm.com> Content-Type: text/plain; charset="ISO-8859-15" Organization: Kihon Technologies Inc. Date: Wed, 31 Mar 2010 22:25:53 -0400 Message-ID: <1270088753.19685.8218.camel@gandalf.stny.rr.com> Mime-Version: 1.0 X-Mailer: Evolution 2.28.2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2010-03-31 at 19:13 -0700, Darren Hart wrote: > 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... I'm talking about the original idea people had of "lets spin for 50us and hope it is unlocked before then", which I thought was not a good idea. > > > > >> 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. OK, that's good. > > > 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();) performance penalties compared to what? not having adaptive at all? > > > 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. Sure it's not a problem ;-) -- Steve