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 S1751685AbdG0NOY (ORCPT <rfc822;w@1wt.eu>);
        Thu, 27 Jul 2017 09:14:24 -0400
Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:43639 "EHLO
        mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL)
        by vger.kernel.org with ESMTP id S1751705AbdG0NOX (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 27 Jul 2017 09:14:23 -0400
Date: Thu, 27 Jul 2017 06:14:18 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.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
Reply-To: paulmck@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>
 <1415161894.26359.1501092075006.JavaMail.zimbra@efficios.com>
 <20170727085312.xweucokjghay3jtx@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170727085312.xweucokjghay3jtx@hirez.programming.kicks-ass.net>
User-Agent: Mutt/1.5.21 (2010-09-15)
X-TM-AS-GCONF: 00
x-cbid: 17072713-0052-0000-0000-000002467C8A
X-IBM-SpamModules-Scores: 
X-IBM-SpamModules-Versions: BY=3.00007435; HX=3.00000241; KW=3.00000007;
 PH=3.00000004; SC=3.00000214; SDB=6.00893642; UDB=6.00446783; IPR=6.00673803;
 BA=6.00005495; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000;
 ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00016408; XFM=3.00000015;
 UTC=2017-07-27 13:14:21
X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused
x-cbparentid: 17072713-0053-0000-0000-000051783E19
Message-Id: <20170727131418.GR3730@linux.vnet.ibm.com>
X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-07-27_06:,,
 signatures=0
X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0
 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam
 adjust=0 reason=mlx scancount=1 engine=8.0.1-1706020000
 definitions=main-1707270208
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Jul 27, 2017 at 10:53:12AM +0200, Peter Zijlstra wrote:
> On Wed, Jul 26, 2017 at 06:01:15PM +0000, Mathieu Desnoyers wrote:
> 
> > 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.
> 
> You forgot to ask yourself how you could abuse this.. just spawn more
> threads.
> 
> Per-thread limits are nearly useless, because spawning new threads is
> cheap.

Agreed -- any per-thread limit has to be some portion/fraction of a
global limit.

							Thanx, Paul