From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755763AbYHYPMq (ORCPT ); Mon, 25 Aug 2008 11:12:46 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752931AbYHYPMg (ORCPT ); Mon, 25 Aug 2008 11:12:36 -0400 Received: from e31.co.us.ibm.com ([32.97.110.149]:48881 "EHLO e31.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752740AbYHYPMf (ORCPT ); Mon, 25 Aug 2008 11:12:35 -0400 Date: Mon, 25 Aug 2008 08:12:20 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Christoph Lameter , Pekka Enberg , Ingo Molnar , Jeremy Fitzhardinge , Nick Piggin , Andi Kleen , "Pallipadi, Venkatesh" , Suresh Siddha , Jens Axboe , Rusty Russell , Linux Kernel Mailing List Subject: Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu Message-ID: <20080825151220.GA6745@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <84144f020808220006n25d684b1n9db306ddc4f58c4c@mail.gmail.com> <48AEC6B2.1080701@linux-foundation.org> <20080822151156.GA6744@linux.vnet.ibm.com> <48AEF3FD.70906@linux-foundation.org> <20080822182915.GG6744@linux.vnet.ibm.com> <48AF0735.60402@linux-foundation.org> <20080822195226.GJ6744@linux.vnet.ibm.com> <48AF1B81.3050806@linux-foundation.org> <20080822205339.GK6744@linux.vnet.ibm.com> <1219660291.8515.20.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1219660291.8515.20.camel@twins> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Aug 25, 2008 at 12:31:31PM +0200, Peter Zijlstra wrote: > On Fri, 2008-08-22 at 13:53 -0700, Paul E. McKenney wrote: > > On Fri, Aug 22, 2008 at 03:03:13PM -0500, Christoph Lameter wrote: > > > Paul E. McKenney wrote: > > > > > > > I was indeed thinking in terms of the free from RCU being specially marked. > > > > > > Isnt there some way to shorten the rcu periods significantly? Critical > > > sections do not take that long after all. > > > > In theory, yes. However, the shorter the grace period, the greater the > > per-update overhead of grace-period detection -- the general approach > > is to use a per-CPU high-resolution timer to force RCU grace period > > processing every 100 microseconds or so. > > You could of course also drive the rcu state machine from > rcu_read_unlock(). True, and Jim Houston implemented something similar to this some years back: http://marc.theaimsgroup.com/?l=linux-kernel&m=109387402400673&w=2 This of course greatly increases rcu_read_unlock() overhead. But perhaps it is a good implementation for the workloads that Christoph is thinking of. > > Also, by definition, the RCU > > grace period can be no shorter than the longest active RCU read-side > > critical section. Nevertheless, I have designed my current hierarchical > > RCU patch with expedited grace periods in mind, though more for the > > purpose of reducing latency of long strings of operations that involve > > synchronize_rcu() than for cache locality. > > Another thing that could be done is more often force a grace period by > flipping the counters. Yep. That is exactly what I was getting at with the high-resolution timer point above. This seems to be a reasonable compromise, as it allows someone to specify how quickly the grace periods happen dynamically. But I am not sure that this gets the grace periods to go fast enough to cover Christoph's use case -- he seems to be in a "faster is better" space rather than in an "at least this fast" space. Still, it would likely help in some important cases. Thanx, Paul