From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754038Ab0CDR5G (ORCPT ); Thu, 4 Mar 2010 12:57:06 -0500 Received: from mail.openrapids.net ([64.15.138.104]:35579 "EHLO blackscsi.openrapids.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753117Ab0CDR5E (ORCPT ); Thu, 4 Mar 2010 12:57:04 -0500 Date: Thu, 4 Mar 2010 12:56:59 -0500 From: Mathieu Desnoyers To: Linus Torvalds Cc: Ingo Molnar , 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: <20100304175659.GA3255@Krystal> References: <20100225232316.GA30196@Krystal> <20100304122304.GA6864@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Editor: vi X-Info: http://www.efficios.com X-Operating-System: Linux/2.6.26-2-686 (i686) X-Uptime: 12:40:55 up 40 days, 20:18, 5 users, load average: 0.40, 0.35, 0.48 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 * 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. Thanks, Mathieu -- Mathieu Desnoyers Operating System Efficiency Consultant EfficiOS Inc. http://www.efficios.com