From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751210AbdGYSxm (ORCPT ); Tue, 25 Jul 2017 14:53:42 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:55704 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750973AbdGYSxj (ORCPT ); Tue, 25 Jul 2017 14:53:39 -0400 Date: Tue, 25 Jul 2017 20:53:20 +0200 From: Peter Zijlstra To: "Paul E. McKenney" 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 Message-ID: <20170725185320.uis4hxqaqlx7y7gp@hirez.programming.kicks-ass.net> 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> <20170725171701.GS3730@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170725171701.GS3730@linux.vnet.ibm.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 25, 2017 at 10:17:01AM -0700, Paul E. McKenney wrote: > > 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. That doesn't sound right. munmap() of a shared file only invalidates this process's map of it. Swapping a file page otoh will indeed touch the union of cpumasks over all processes mapping that page. > 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. Which are those? I thought we significantly reduced those with the nohz full work. Most IPI uses now first check if a CPU actually needs the IPI before sending it IIRC. > 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. Sending IPIs to mm_cpumask() might be better than expedited, but I'm still hesitant. Just because people want it doesn't mean its a good idea. We need to weight this against the potential for abuse. People want userspace preempt disable, no matter how hard they want it, they're not getting it because its a completely crap idea.