From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753558AbcKGQ7S (ORCPT ); Mon, 7 Nov 2016 11:59:18 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:52546 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753223AbcKGQ7P (ORCPT ); Mon, 7 Nov 2016 11:59:15 -0500 Date: Mon, 7 Nov 2016 17:55:47 +0100 (CET) From: Thomas Gleixner To: Chris Metcalf cc: Gilad Ben Yossef , Steven Rostedt , Ingo Molnar , Peter Zijlstra , Andrew Morton , Rik van Riel , Tejun Heo , Frederic Weisbecker , "Paul E. McKenney" , Christoph Lameter , Viresh Kumar , Catalin Marinas , Will Deacon , Andy Lutomirski , Daniel Lezcano , Francis Giraldeau , Andi Kleen , Arnd Bergmann , linux-kernel@vger.kernel.org Subject: Re: task isolation discussion at Linux Plumbers In-Reply-To: <1605b087-2b3b-77c1-01ac-084e378f5f28@mellanox.com> Message-ID: References: <1471382376-5443-1-git-send-email-cmetcalf@mellanox.com> <1605b087-2b3b-77c1-01ac-084e378f5f28@mellanox.com> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 5 Nov 2016, Chris Metcalf wrote: > == Remote statistics == > > We discussed the possibility of remote statistics gathering, i.e. load > average etc. The idea would be that we could have housekeeping > core(s) periodically iterate over the nohz cores to load their rq > remotely and do update_current etc. Presumably it should be possible > for a single housekeeping core to handle doing this for all the > nohz_full cores, as we only need to do it quite infrequently. > > Thomas suggested that this might be the last remaining thing that > needed to be done to allow disabling the current behavior of falling > back to a 1 Hz clock in nohz_full. > > I believe Thomas said he had a patch to do this already. No, Riek was working on that. > == Remote LRU cache drain == > > One of the issues with task isolation currently is that the LRU cache > drain must be done prior to entering userspace, but it requires > interrupts enabled and thus can't be done atomically. My previous > patch series have handled this by checking with interrupts disabled, > but then looping around with interrupts enabled to try to drain the > LRU pagevecs. Experimentally this works, but it's not provable that > it terminates, which is worrisome. Andy suggested adding a percpu > flag to disable creation of deferred work like LRU cache pages. > > Thomas suggested using an RT "local lock" to guard the LRU cache > flush; he is planning on bringing the concept to mainline in any case. > However, after some discussion we converged on simply using a spinlock > to guard the appropriate resources. As a result, the > lru_add_drain_all() code that currently queues work on each remote cpu > to drain it, can instead simply acquire the lock and drain it remotely. > This means that a task isolation task no longer needs to worry about > being interrupted by SMP function call IPIs, so we don't have to deal > with this in the task isolation framework any more. > > I don't recall anyone else volunteering to tackle this, so I will plan > to look at it. The patch to do that should be orthogonal to the > revised task isolation patch series. I offered to clean up the patch from RT. I'll do that in the next days. > == Missing oneshot_stopped callbacks == > > I raised the issue that various clock_event_device sources don't > always support oneshot_stopped, which can cause an additional > final interrupt to occur after the timer infrastructure believes the > interrupt has been stopped. I have patches to fix this for tile and > arm64 in my patch series; Thomas volunteered to look at adding > equivalent support for x86. Right. Thanks, tglx