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

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

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.