linux-kernel - Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets

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Date:	Mon, 20 Feb 2012 13:49:05 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Alan Stern <stern@...land.harvard.edu>,
	paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...e.hu>,
	paul@...lmenage.org, tj@...nel.org, frank.rowand@...sony.com,
	pjt@...gle.com, tglx@...utronix.de, lizf@...fujitsu.com,
	prashanth@...ux.vnet.ibm.com, vatsa@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling
 related to cpusets

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.

> > At the moment, I am exploring several ways to achieve this (I can think of 2
> > ways at the moment, will see which one is better). But in case this approach
> > itself seems wrong for any reason, please let me know.


Have you actually tried the simple patch? 

Calling partition_sched_domains() like you do doesn't seem right, it
completely ignores cpusets, it will make certain cpuset configurations
mis-behave.
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