From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner+w=401wt.eu-S932476Ab0AGIpM@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S932476Ab0AGIpM (ORCPT <rfc822;w@1wt.eu>);
	Thu, 7 Jan 2010 03:45:12 -0500
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932227Ab0AGIpL
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Thu, 7 Jan 2010 03:45:11 -0500
Received: from casper.infradead.org ([85.118.1.10]:47838 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S932187Ab0AGIpJ (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 7 Jan 2010 03:45:09 -0500
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
 barrier
From: Peter Zijlstra <peterz@infradead.org>
To: Josh Triplett <josh@joshtriplett.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Ingo Molnar <mingo@elte.hu>, akpm@linux-foundation.org,
       tglx@linutronix.de, Valdis.Kletnieks@vt.edu, dhowells@redhat.com,
       laijs@cn.fujitsu.com, dipankar@in.ibm.com
In-Reply-To: <20100107063558.GC12939@feather>
References: <20100107044007.GA22863@Krystal>
	 <1262842854.28171.3710.camel@gandalf.stny.rr.com>
	 <20100107061955.GC25786@Krystal>  <20100107063558.GC12939@feather>
Content-Type: text/plain; charset="UTF-8"
Date: Thu, 07 Jan 2010 09:44:15 +0100
Message-ID: <1262853855.4049.86.camel@laptop>
Mime-Version: 1.0
X-Mailer: Evolution 2.28.1 
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, 2010-01-06 at 22:35 -0800, Josh Triplett wrote:
> 
> The number of threads doesn't matter nearly as much as the number of
> threads typically running at a time compared to the number of
> processors.  Of course, we can't measure that as easily, but I don't
> know that your proposed heuristic would approximate it well.

Quite agreed, and not disturbing RT tasks is even more important.

A simple:

  for_each_cpu(cpu, current->mm->cpu_vm_mask) {
     if (cpu_curr(cpu)->mm == current->mm)
        smp_call_function_single(cpu, func, NULL, 1);
  }

seems far preferable over anything else, if you really want you can use
a cpumask to copy cpu_vm_mask in and unset bits and use the mask with
smp_call_function_any(), but that includes having to allocate the
cpumask, which might or might not be too expensive for Mathieu.