From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751724AbdGYRRI (ORCPT ); Tue, 25 Jul 2017 13:17:08 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:60161 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751210AbdGYRRH (ORCPT ); Tue, 25 Jul 2017 13:17:07 -0400 Date: Tue, 25 Jul 2017 10:17:01 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.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> <1500933497-12612-4-git-send-email-paulmck@linux.vnet.ibm.com> <20170725163318.bporqvcoodtel4a6@hirez.programming.kicks-ass.net> <20170725164900.GR3730@linux.vnet.ibm.com> <20170725165957.alykngbnrrwn3onw@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170725165957.alykngbnrrwn3onw@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 17072517-0040-0000-0000-00000385824B X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00007424; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000214; SDB=6.00892763; UDB=6.00446256; IPR=6.00672927; BA=6.00005492; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00016372; XFM=3.00000015; UTC=2017-07-25 17:17:04 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17072517-0041-0000-0000-00000779A05B Message-Id: <20170725171701.GS3730@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-07-25_08:,, 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-1707250271 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 25, 2017 at 06:59:57PM +0200, Peter Zijlstra wrote: > On Tue, Jul 25, 2017 at 09:49:00AM -0700, Paul E. McKenney wrote: > > On Tue, Jul 25, 2017 at 06:33:18PM +0200, Peter Zijlstra wrote: > > > On Mon, Jul 24, 2017 at 02:58:16PM -0700, Paul E. McKenney wrote: > > > > The sys_membarrier() system call has proven too slow for some use > > > > cases, which has prompted users to instead rely on TLB shootdown. > > > > Although TLB shootdown is much faster, it has the slight disadvantage > > > > of not working at all on arm and arm64. This commit therefore adds > > > > an expedited option to the sys_membarrier() system call. > > > > > > > @@ -64,6 +65,10 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags) > > > > if (num_online_cpus() > 1) > > > > synchronize_sched(); > > > > return 0; > > > > + case MEMBARRIER_CMD_SHARED_EXPEDITED: > > > > + if (num_online_cpus() > 1) > > > > + synchronize_sched_expedited(); > > > > + return 0; > > > > > > So you now give unprivileged userspace the means to IPI the entire > > > machine? > > > > > > So what do we do when someone goes and does: > > > > > > for (;;) > > > sys_membarrier(MEMBARRIER_CMD_SHARED_EXPEDITED, 0); > > > > > > on us? > > > > The same thing that happens when they call munmap(). > > munmap() TLB invalidate is limited to those CPUs that actually ran > threads of their process, while this is machine wide. Or those CPUs running threads of any process mapping the underlying file or whatever. And in either case, this can span the whole machine. Plus there are a number of other ways for users to do on-demand full-system IPIs, including any number of ways to wake up large numbers of CPUs, including from unrelated processes. But I do plan to add another alternative that is limited to threads of the running process. I will be carrying both versions to enable those who have been bugging me about this to do testing. Thanx, Paul