From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751427AbdGZR7L (ORCPT ); Wed, 26 Jul 2017 13:59:11 -0400 Received: from mail.efficios.com ([167.114.142.141]:39321 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750867AbdGZR7K (ORCPT ); Wed, 26 Jul 2017 13:59:10 -0400 Date: Wed, 26 Jul 2017 18:01:15 +0000 (UTC) From: Mathieu Desnoyers To: "Paul E. McKenney" Cc: Peter Zijlstra , linux-kernel , Ingo Molnar , Lai Jiangshan , dipankar , Andrew Morton , Josh Triplett , Thomas Gleixner , rostedt , David Howells , Eric Dumazet , fweisbec , Oleg Nesterov , Will Deacon Message-ID: <1415161894.26359.1501092075006.JavaMail.zimbra@efficios.com> In-Reply-To: <20170726154229.GO3730@linux.vnet.ibm.com> References: <20170724215758.GA12075@linux.vnet.ibm.com> <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> <20170726074656.obadfdu6hdlrmy7r@hirez.programming.kicks-ass.net> <20170726154229.GO3730@linux.vnet.ibm.com> Subject: Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.142.141] X-Mailer: Zimbra 8.7.9_GA_1794 (ZimbraWebClient - FF52 (Linux)/8.7.9_GA_1794) Thread-Topic: sys_membarrier: Add expedited option Thread-Index: /4GRpvjnpbRP+LuPWkAA/OoDtBnHMA== Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ----- On Jul 26, 2017, at 11:42 AM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote: > On Wed, Jul 26, 2017 at 09:46:56AM +0200, Peter Zijlstra wrote: >> 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). > > Agreed, we should keep things simple to start with. The user can always > invoke sys_membarrier() from each process. Another alternative for a MEMBARRIER_CMD_SHARED_EXPEDITED would be rate-limiting per thread. For instance, we could add a new "ulimit" that would bound the number of expedited membarrier per thread that can be done per millisecond, and switch to synchronize_sched() whenever a thread goes beyond that limit for the rest of the time-slot. A RT system that really cares about not having userspace sending IPIs to all cpus could set the ulimit value to 0, which would always use synchronize_sched(). Thoughts ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com