From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752031AbcFUUlO (ORCPT ); Tue, 21 Jun 2016 16:41:14 -0400 Received: from merlin.infradead.org ([205.233.59.134]:46358 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751892AbcFUUko (ORCPT ); Tue, 21 Jun 2016 16:40:44 -0400 Date: Tue, 21 Jun 2016 21:37:09 +0200 From: Peter Zijlstra To: Tejun Heo Cc: Gautham R Shenoy , Thomas Gleixner , Michael Ellerman , Abdul Haleem , Aneesh Kumar , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 1/2] workqueue: Move wq_update_unbound_numa() to the beginning of CPU_ONLINE Message-ID: <20160621193709.GT30154@twins.programming.kicks-ass.net> References: <6b3c7059ec5d2d6157d23d619e4507692a42a5bd.1465311052.git.ego@linux.vnet.ibm.com> <20160615155350.GB24102@mtj.duckdns.org> <20160615192844.GA20301@in.ibm.com> <20160616193504.GB3262@mtj.duckdns.org> <20160621141231.GA30251@in.ibm.com> <20160621153651.GF3262@mtj.duckdns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160621153651.GF3262@mtj.duckdns.org> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 21, 2016 at 11:36:51AM -0400, Tejun Heo wrote: > On Tue, Jun 21, 2016 at 07:42:31PM +0530, Gautham R Shenoy wrote: > > > Subject: [PATCH] sched: allow kthreads to fallback to online && !active cpus > > > > > > During CPU hotplug, CPU_ONLINE callbacks are run while the CPU is > > > online but not active. A CPU_ONLINE callback may create or bind a > > > kthread so that its cpus_allowed mask only allows the CPU which is > > > being brought online. The kthread may start executing before the CPU > > > is made active and can end up in select_fallback_rq(). > > > > > > In such cases, the expected behavior is selecting the CPU which is > > > coming online; however, because select_fallback_rq() only chooses from > > > active CPUs, it determines that the task doesn't have any viable CPU > > > in its allowed mask and ends up overriding it to cpu_possible_mask. > > > > > > CPU_ONLINE callbacks should be able to put kthreads on the CPU which > > > is coming online. Update select_fallback_rq() so that it follows > > > cpu_online() rather than cpu_active() for kthreads. > > > > > > Signed-off-by: Tejun Heo > > > Reported-by: Gautham R Shenoy > > > > Hi Tejun, > > > > This patch fixes the issue on POWER. I am able to see the worker > > threads of the unbound workqueues of the newly onlined node with this. > > > > Tested-by: Gautham R. Shenoy > > Peter? Hurm.. So I've applied it, just to get this issue sorted, but I'm not entirely sure I like it. I think I prefer ego's version because that makes it harder to get stuff to run on !active,online cpus. I think we really want to be careful what gets to run during that state.