Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756079AbYGYNjr (ORCPT ); Fri, 25 Jul 2008 09:39:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752712AbYGYNji (ORCPT ); Fri, 25 Jul 2008 09:39:38 -0400 Received: from viefep20-int.chello.at ([62.179.121.40]:42727 "EHLO viefep20-int.chello.at" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752423AbYGYNji (ORCPT ); Fri, 25 Jul 2008 09:39:38 -0400 X-SourceIP: 62.163.52.83 Subject: Re: [patch, rfc: 2/2] sched, hotplug: ensure a task is on the valid cpu after set_cpus_allowed_ptr() From: Peter Zijlstra To: Dmitry Adamushko Cc: Ingo Molnar , LKML In-Reply-To: References: <1216937730.5368.16.camel@earth> <1216989649.7257.381.camel@twins> Content-Type: text/plain Date: Fri, 25 Jul 2008 15:39:44 +0200 Message-Id: <1216993184.7257.388.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.22.3.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2885 Lines: 72 On Fri, 2008-07-25 at 15:20 +0200, Dmitry Adamushko wrote: > 2008/7/25 Peter Zijlstra : > > On Fri, 2008-07-25 at 00:15 +0200, Dmitry Adamushko wrote: > >> > >> From: Dmitry Adamushko > >> Subject: sched, hotplug: ensure a task is on the valid cpu after > >> set_cpus_allowed_ptr() > >> > >> --- > >> sched, hotplug: ensure a task is on the valid cpu after set_cpus_allowed_ptr() > >> > >> The 'new_mask' may not include task_cpu(p) so we migrate 'p' on another 'cpu'. > >> In case it can't be placed on this 'cpu' immediately, we submit a request > >> to the migration thread and wait for its completion. > >> > >> Now, by the moment this request gets handled by the migration_thread, > >> 'cpu' may well be offline/non-active. As a result, 'p' continues > >> running on its old cpu which is not in the 'new_mask'. > >> > >> Fix it: ensure 'p' ends up on a valid cpu. > >> > >> Theoreticaly (but unlikely), we may get an endless loop if someone cpu_down()'s > >> a new cpu we have choosen on each iteration. > >> > >> Alternatively, we may introduce a special type of request to migration_thread, > >> namely "move_to_any_allowed_cpu" (e.g. by specifying dest_cpu == -1). > >> > >> Note, any_active_cpu() instead of any_online_cpu() would be better here. > > > > Hrmm,.. this is all growing into something of a mess.. defeating the > > whole purpose of introducing that cpu_active_map stuff. > > > > Would the suggested SRCU logic simplify all this? > > Ah, wait a second. > > sched_setaffinity() -> set_cpus_allowed_ptr() is ok vs. cpu_down() as > it does use get_online_cpus(). So none of the cpus can become offline > while we are in set_cpus_allowed_ptr(). > > but there are numerous calls to set_cpus_allowed_ptr() from other > places and not all of them seem to call get_online_cpus()... > > yeah, I should check this issue again.. > > btw., indeed all these different sync. cases are a bit of mess. Will ponder it a bit more, but my brain can't seem to let go of SRCU now.. I'll go concentrate on making the swap-over-nfs patches prettier, maybe that will induce a brainwave ;-) > --- > > btw., I was wondering about this change: > > ba42059fbd0aa1ac91b582412b5fedb1258f241f > > sched: hrtick_enabled() should use cpu_active() > > Peter pointed out that hrtick_enabled() should use cpu_active(). What exactly were you wondering about? It seemed a good idea to stop starting hrtimers before we migrate them to another cpu (one of the things done later in cpu_down), thereby avoiding spurious fires on remote cpus. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/