From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936719Ab0COUxR (ORCPT ); Mon, 15 Mar 2010 16:53:17 -0400 Received: from mail.openrapids.net ([64.15.138.104]:48610 "EHLO blackscsi.openrapids.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932183Ab0COUxP (ORCPT ); Mon, 15 Mar 2010 16:53:15 -0400 Date: Mon, 15 Mar 2010 16:53:12 -0400 From: Mathieu Desnoyers To: Ingo Molnar Cc: Linus Torvalds , KOSAKI Motohiro , Steven Rostedt , "Paul E. McKenney" , Nicholas Miell , laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, josh@joshtriplett.org, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, linux-kernel@vger.kernel.org, Nick Piggin , Chris Friesen , Fr??d??ric Weisbecker Subject: Re: [PATCH -tip] introduce sys_membarrier(): process-wide memory barrier (v9) Message-ID: <20100315205312.GA31231@Krystal> References: <20100225232316.GA30196@Krystal> <20100304122304.GA6864@elte.hu> <20100304175659.GA3255@Krystal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100304175659.GA3255@Krystal> X-Editor: vi X-Info: http://www.efficios.com X-Operating-System: Linux/2.6.26-2-686 (i686) X-Uptime: 16:46:17 up 51 days, 23:23, 5 users, load average: 0.07, 0.06, 0.08 User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Mathieu Desnoyers (mathieu.desnoyers@efficios.com) wrote: > * Linus Torvalds (torvalds@linux-foundation.org) wrote: > > > - SA_RUNNING: a way to signal only running threads - as a way for user-space > > > based concurrency control mechanisms to deschedule running threads (or, like > > > in your case, to implement barrier / garbage collection schemes). > > > > Hmm. This sounds less fundamentally broken, but at the same time also > > _way_ more invasive in the signal handling layer. It's already one of our > > more "exciting" layers out there. > > > > Hrm, thinking about it a bit further, the only way I see we could provide a > usable SA_RUNNING flag would be to add hooks to the scheduler. These hooks would > somehow have to call user-space code (!) when scheduling in/out a thread. Yes, > this sounds utterly broken (since these hooks would have to be preemptable). > > The idea is this: if we look, for instance, at the kernel preemptable RCU > implementations, they consist of two parts: one is iteration on all CPUs to > consider all active CPUs, and the other is a modification of the scheduler to > note all preempted tasks that were in a preemptable RCU C.S.. > > Just for the memory barrier we consider for sys_membarrier(), I had to ensure > that the scheduler issues memory barriers to order accesses to user-space memory > and mm_cpumask modifications. In reality, what we are doing is to ensure that > the operation required on the running thread is done by the scheduler too when > scheduling in/out the task. > > As soon as we have signal handlers which perform more than a simple memory > barrier (e.g. something that has side-effects outside of the processor), I doubt > it would ever make sense to only run the handler on running threads unless we > have hooks in the scheduler too. Unless this question is answered, Ingo's SA_RUNNING signal proposal, as appealing as it may look at a first glance, falls into the "fundamentally broken" category. I don't see any neat way to make the scheduler call into user-space hooks to deal with inherent synchronization required between iteration on active threads and scheduler activity. But who knows, maybe it's just a lack of imagination from my part. Thanks, Mathieu > > Thanks, > > Mathieu > > -- > Mathieu Desnoyers > Operating System Efficiency Consultant > EfficiOS Inc. > http://www.efficios.com -- Mathieu Desnoyers Operating System Efficiency Consultant EfficiOS Inc. http://www.efficios.com