From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757450AbdDFLD0 (ORCPT ); Thu, 6 Apr 2017 07:03:26 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:36579 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757012AbdDFLDS (ORCPT ); Thu, 6 Apr 2017 07:03:18 -0400 Date: Thu, 6 Apr 2017 13:03:16 +0200 (CEST) From: Thomas Gleixner To: Peter Zijlstra cc: Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org, Mike Galbraith , Ingo Molnar , "Rafael J . Wysocki" Subject: Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU mask In-Reply-To: <20170406105736.uw2j22igja45wi4b@hirez.programming.kicks-ass.net> Message-ID: References: <20170404184202.20376-1-bigeasy@linutronix.de> <20170406093224.q3tunb6gdx5a6flr@hirez.programming.kicks-ass.net> <20170406094626.a42cq66ezgq4lohh@linutronix.de> <20170406103516.i3jyjcmee65r3wwj@hirez.programming.kicks-ass.net> <20170406105736.uw2j22igja45wi4b@hirez.programming.kicks-ass.net> 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 Thu, 6 Apr 2017, Peter Zijlstra wrote: > On Thu, Apr 06, 2017 at 12:47:21PM +0200, Thomas Gleixner wrote: > > On Thu, 6 Apr 2017, Peter Zijlstra wrote: > > > > IIRC MIPS has a case where only 1 in N cores has an FPU. And once a task > > > uses FPU, it gets affined to the core that has one or something like > > > that. > > > > > > Of course, nothing then stops someone else breaking that affinity. But I > > > suspect it will simply fault on the next FPU instruction and 'reset' the > > > mask or something. I've no clue and no real desire to know. > > > > It does nasty games with it's own storage of p->thread.user_cpus_allowed > > and a fully seperate implementation of sys_sched_set|getaffinity. > > > > Plus a magic trap handler which forces the thread to a CPU with FPU when > > the user_cpus_allowed mask intersects with the cpus_with_fpu_mask... > > > > Magic crap, which could all be replaced by a simple function in the > > scheduler which allows to push a task to a FPU CPU and then disable > > migration. > > If its even halfway coherent, I'd much rather let it stay where it it. > > I really want to limit migrate_disable() to PREEMPT_RT=y where its used > to preserve spinlock semantics and not allow random other > migrate_disable() usage in the kernel. > > Also note, that per the above, it can actually migrate to any core that > has an FPU on, so its not a good match for migrate_disable() in any > case. Fair enough, but I prefer to have the ability for a temporary restriction of the user space visible cpus allowed mask in the core code, which can be used for both migrate_disable() and things like that MIPS FPU stuff rather than all those home brewn hackeries which are prone to bitrot, security issues and subtle wreckage. cpus_allowed should be solely under sched core control and not accessible from anything outside. Thanks, tglx