From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756813AbdDFIBt (ORCPT ); Thu, 6 Apr 2017 04:01:49 -0400 Received: from mail-wr0-f193.google.com ([209.85.128.193]:34011 "EHLO mail-wr0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752503AbdDFIBo (ORCPT ); Thu, 6 Apr 2017 04:01:44 -0400 Date: Thu, 6 Apr 2017 10:01:39 +0200 From: Ingo Molnar To: Sebastian Andrzej Siewior Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Thomas Gleixner , Mike Galbraith , Ingo Molnar , "Rafael J . Wysocki" Subject: Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU mask Message-ID: <20170406080139.GA22069@gmail.com> References: <20170404184202.20376-1-bigeasy@linutronix.de> <20170405073943.GA17266@gmail.com> <20170405083753.7eszej2njds4ovdb@linutronix.de> <20170406061622.GA19979@gmail.com> <20170406073832.e7bu4ldpfuq44ui6@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170406073832.e7bu4ldpfuq44ui6@linutronix.de> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Sebastian Andrzej Siewior wrote: > On 2017-04-06 08:16:22 [+0200], Ingo Molnar wrote: > > > > * Sebastian Andrzej Siewior wrote: > > > > > On 2017-04-05 09:39:43 [+0200], Ingo Molnar wrote: > > > > > > > > So maybe we could add the following facility: > > > > > > > > ptr = sched_migrate_to_cpu_save(cpu); > > > > > > > > ... > > > > > > > > sched_migrate_to_cpu_restore(ptr); > > > > BTW., and I'm sure this has come up before, but why doesn't migrate_disable() use > > a simple per task flag that the scheduler migration code takes into account? > > we could add that. But right now there are two spots which look at the > counter to decide whether or not migration is disabled. > > > It should be functionally equivalent to the current solution, and it appears to > > have a heck of a smaller cross section with the rest of the scheduler. > > > > I.e.: > > > > static inline void migrate_disable(void) > > { > > current->migration_disabled++; > > } > > > > ... > > > > static inline void migrate_enable(void) > > { > > current->migration_disabled--; > > } > > > > or so? Then add this flag as a condition to can_migrate_task() et al. > > > > While we generally dislike such flags as they wreck havoc with the scheduler if > > overused, the cpus_allowed based solution has the exact same effect so it's not > > like it's a step backwards - and it should also be much faster and less intrusive. > > So you are saying that we drop the cpus_ptr + cpus_mask fields again and > instead add a task-flag to ensure that the scheduler does not migrate > the task to another CPU? Yeah - but no need to add a per-task flag if we already have a counter. > > Am I missing some complication? > > We do have the counter. We have need to ensure that the CPU is not going away > while we are in a migrate_disable() region since we can be scheduled out. So the > CPU can't go offline until we leave that region. Yeah. But it should be relatively straightforward to extend the logic that makes sure that a CPU does not go away from under tasks pinned to that CPU alone, right? > #define migrate_disable() sched_migrate_to_cpu_save(-1) > > int sched_migrate_to_cpu_save(int cpu) So if we have a ->migration_disabled counter then we don't need the sched_migrate_to_cpu_save()/restore() complication, right? Sorry if this is a back and forth - I was somehow convinced that we do need to frob the cpus_allowed mask to get this functionality - but in hindsight I think the counter should be enough. I.e. just have a counter and these two APIs: static inline void migrate_disable(void) { current->migration_disabled++; } ... static inline void migrate_enable(void) { current->migration_disabled--; } ... and make sure the scheduler migration code plus the CPU hotplug code considers the counter. Would this work, and would this be the simplest all around solution? Thanks, Ingo