From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751228AbdGZHrN (ORCPT <rfc822;w@1wt.eu>);
        Wed, 26 Jul 2017 03:47:13 -0400
Received: from bombadil.infradead.org ([65.50.211.133]:56655 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750813AbdGZHrM (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 26 Jul 2017 03:47:12 -0400
Date: Wed, 26 Jul 2017 09:46:56 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Lai Jiangshan <jiangshanlai@gmail.com>,
        dipankar <dipankar@in.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>, rostedt <rostedt@goodmis.org>,
        David Howells <dhowells@redhat.com>,
        Eric Dumazet <edumazet@google.com>, fweisbec <fweisbec@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option
Message-ID: <20170726074656.obadfdu6hdlrmy7r@hirez.programming.kicks-ass.net>
References: <20170724215758.GA12075@linux.vnet.ibm.com>
 <20170725165957.alykngbnrrwn3onw@hirez.programming.kicks-ass.net>
 <20170725171701.GS3730@linux.vnet.ibm.com>
 <20170725185320.uis4hxqaqlx7y7gp@hirez.programming.kicks-ass.net>
 <20170725193612.GW3730@linux.vnet.ibm.com>
 <20170725202451.GC28975@worktop>
 <20170725211926.GA3730@linux.vnet.ibm.com>
 <20170725215510.GD28975@worktop>
 <1480480872.25829.1501023013862.JavaMail.zimbra@efficios.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1480480872.25829.1501023013862.JavaMail.zimbra@efficios.com>
User-Agent: NeoMutt/20170609 (1.8.3)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, Jul 25, 2017 at 10:50:13PM +0000, Mathieu Desnoyers wrote:
> This would implement a MEMBARRIER_CMD_PRIVATE_EXPEDITED (or such) flag
> for expedited process-local effect. This differs from the "SHARED" flag,
> since the SHARED flag affects threads accessing memory mappings shared
> across processes as well.
> 
> I wonder if we could create a MEMBARRIER_CMD_SHARED_EXPEDITED behavior
> by iterating on all memory mappings mapped into the current process,
> and build a cpumask based on the union of all mm masks encountered ?
> Then we could send the IPI to all cpus belonging to that cpumask. Or
> am I missing something obvious ?

I would readily object to such a beast. You far too quickly end up
having to IPI everybody because of some stupid shared map or something
(yes I know, normal DSOs are mapped private).

The whole private_expedited is only palatable because we can only hinder
our own threads (much).