From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756897Ab2BXXUT (ORCPT ); Fri, 24 Feb 2012 18:20:19 -0500 Received: from ogre.sisk.pl ([217.79.144.158]:41626 "EHLO ogre.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756200Ab2BXXUR (ORCPT ); Fri, 24 Feb 2012 18:20:17 -0500 From: "Rafael J. Wysocki" To: "Srivatsa S. Bhat" Subject: Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets Date: Sat, 25 Feb 2012 00:24:17 +0100 User-Agent: KMail/1.13.6 (Linux/3.3.0-rc4+; KDE/4.6.0; x86_64; ; ) Cc: Peter Zijlstra , Alan Stern , paulmck@linux.vnet.ibm.com, Ingo Molnar , paul@paulmenage.org, tj@kernel.org, frank.rowand@am.sony.com, pjt@google.com, tglx@linutronix.de, lizf@cn.fujitsu.com, prashanth@linux.vnet.ibm.com, vatsa@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, "akpm@linux-foundation.org" References: <4F4243B6.8070507@linux.vnet.ibm.com> <4F460D7B.1020703@linux.vnet.ibm.com> In-Reply-To: <4F460D7B.1020703@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201202250024.17644.rjw@sisk.pl> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thursday, February 23, 2012, Srivatsa S. Bhat wrote: > On 02/20/2012 06:29 PM, Srivatsa S. Bhat wrote: > > > Hi Peter, > > > > On 02/20/2012 06:19 PM, Peter Zijlstra wrote: > > > >> On Fri, 2012-02-17 at 17:45 +0530, Srivatsa S. Bhat wrote: > >> > >>>> Trivially removing CPU_TASKS_FROZEN as shown below doesn't look right to me: > >>>> > >>>> --- > >>>> > >>>> kernel/sched/core.c | 4 ++-- > >>>> 1 files changed, 2 insertions(+), 2 deletions(-) > >>>> > >>>> > >>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c > >>>> index 5255c9d..43a166e 100644 > >>>> --- a/kernel/sched/core.c > >>>> +++ b/kernel/sched/core.c > >>>> @@ -6729,7 +6729,7 @@ int __init sched_create_sysfs_power_savings_entries(struct device *dev) > >>>> static int cpuset_cpu_active(struct notifier_block *nfb, unsigned long action, > >>>> void *hcpu) > >>>> { > >>>> - switch (action & ~CPU_TASKS_FROZEN) { > >>>> + switch (action) { > >>>> case CPU_ONLINE: > >>>> case CPU_DOWN_FAILED: > >>>> cpuset_update_active_cpus(); > >>>> @@ -6742,7 +6742,7 @@ static int cpuset_cpu_active(struct notifier_block *nfb, unsigned long action, > >>>> static int cpuset_cpu_inactive(struct notifier_block *nfb, unsigned long action, > >>>> void *hcpu) > >>>> { > >>>> - switch (action & ~CPU_TASKS_FROZEN) { > >>>> + switch (action) { > >>>> case CPU_DOWN_PREPARE: > >>>> cpuset_update_active_cpus(); > >>>> return NOTIFY_OK; > >>>> > >>>> > >>>> IMO, irrespective of whether we keep cpusets unaware of all CPU Hotplug or > >>>> only unaware of the CPU hotplug in the suspend/resume path, I feel the > >>>> scheduler should always know the true state of the system, ie., offline CPUs > >>>> must not be part of any sched domain, at any point in time. > >> > >> That's really not a problem as long as they're not in the active mask. > >> > > > [...] > > So, based on what you said above, I guess we can go with that simple patch. > (See below, for the patch with changelog). > > I thought about what Ingo suggested (ie., not touching cpusets during cpu > hotplug, irrespective of whether it is part of suspend or not). And we can > implement that by having a scheme something like: > > o Currently if a cpuset's cpus_allowed mask becomes empty due to CPU offline, > all tasks in that cpuset is moved to a parent cpuset whose cpus_allowed mask > is non-empty. > Here, instead of *moving* the tasks to another cpuset, we could just change > the cpus_allowed mask of each task in that cpuset to reflect the non-empty > parent cpuset's cpus_allowed mask. IOW, during a CPU offline, we never touch > a cpuset's cpus_allowed mask, we only modify the cpus_allowed mask of the > *tasks* in that cpuset. Also, we never move a task from one cpuset to another > due to CPU offline. > > o Since we never modify a cpuset's cpus_allowed mask due to CPU offline, it is > trivial to get back to original state when that CPU comes back online. Just > compare the cpuset's cpus_allowed mask with cpu_active_mask and update the > cpus_allowed masks of all the tasks in that cpuset. > > We can definitely do all this, but I am not quite sure if this complexity is > justified (ie., complexity in the sense that the cpus_allowed mask of the tasks > in a cpuset might not always be the same as the cpus_allowed mask of that > cpuset). > > However, if somebody feels that the above mentioned approach looks good and > the complexity is justified, please let me know.. But until then, the > following simple fix for the suspend/resume bug should suffice. > > ---- > > From: Srivatsa S. Bhat > Subject: CPU hotplug, cpusets, suspend: Don't touch cpusets during suspend/resume > > Currently, during CPU hotplug, the cpuset callbacks modify the cpusets > to reflect the state of the system, and this handling is asymmetric. > That is, upon CPU offline, that CPU is removed from all cpusets. However > when it comes back online, it is put back only to the root cpuset. > > This gives rise to a significant problem during suspend/resume. During > suspend, we offline all non-boot cpus and during resume we online them back. > Which means, after a resume, all cpusets (except the root cpuset) will be > restricted to just one single CPU (the boot cpu). But the whole point of > suspend/resume is to restore the system to a state which is as close as > possible to how it was before suspend. > > So to fix this, don't touch cpusets during suspend/resume. That is, modify > the cpuset-related CPU hotplug callback to just ignore CPU hotplug when it > is initiated as part of the suspend/resume sequence. > > Reported-by: Prashanth Nageshappa > Signed-off-by: Srivatsa S. Bhat > Cc: stable@vger.kernel.org So I wonder what people think about this patch. Are there any objections? If not, I'd like to take it for v3.4 until there's a better fix. Thanks, Rafael > --- > > kernel/sched/core.c | 4 ++-- > 1 files changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 1169246..49ba9d4 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -6728,7 +6728,7 @@ int __init sched_create_sysfs_power_savings_entries(struct device *dev) > static int cpuset_cpu_active(struct notifier_block *nfb, unsigned long action, > void *hcpu) > { > - switch (action & ~CPU_TASKS_FROZEN) { > + switch (action) { > case CPU_ONLINE: > case CPU_DOWN_FAILED: > cpuset_update_active_cpus(); > @@ -6741,7 +6741,7 @@ static int cpuset_cpu_active(struct notifier_block *nfb, unsigned long action, > static int cpuset_cpu_inactive(struct notifier_block *nfb, unsigned long action, > void *hcpu) > { > - switch (action & ~CPU_TASKS_FROZEN) { > + switch (action) { > case CPU_DOWN_PREPARE: > cpuset_update_active_cpus(); > return NOTIFY_OK; > > > >