[PATCH v2 7/7] ARM: smp: Add runtime PM support for CPU hotplug

[Rafael J. Wysocki]

On Saturday, September 05, 2015 11:39:20 AM Alan Stern wrote:
> On Sat, 5 Sep 2015, Grygorii Strashko wrote:
> 
> > On 09/04/2015 09:45 PM, Alan Stern wrote:
> > > On Fri, 4 Sep 2015, Grygorii Strashko wrote:
> > > 
> > >> There is one "small" problem with such approach :(
> > >>
> > >> - It's incompatible with -RT kernel, because PM runtime can't be used
> > >> in atomic context on -RT.
> > > 
> > > Can you explain this more fully?  Why can't runtime PM be used in
> > > atomic context in the -rt kernels?
> > >  
> > 
> > See:
> >  http://lwn.net/Articles/146861/
> >  https://rt.wiki.kernel.org/index.php/Frequently_Asked_Questions#How_does_the_CONFIG_PREEMPT_RT_patch_work.3F
> > 
> > spinlock_t
> >     Critical sections are preemptible. The _irq operations (e.g., spin_lock_irqsave())
> >  do -not- disable hardware interrupts. Priority inheritance is used to prevent priority
> >  inversion. An underlying rt_mutex is used to implement spinlock_t in PREEMPT_RT. 
> > 
> > As result, have to do things like:
> > https://lkml.org/lkml/2015/8/18/161
> > https://lkml.org/lkml/2015/8/18/162
> > 
> > Sorry for brief reply - Friday/Sat night :)
> 
> I see.  Although we normally think of interrupt contexts as being
> atomic, in an -rt kernel this isn't true any more because things like
> spin_lock_irq don't actually disable interrupts.
> 
> Therefore it would be correct to say that in -rt kernels, runtime PM
> can be used in interrupt context (if the device is marked as irq-safe),
> but not in atomic context.  Right?

Right.

Whatever is suitable for interrupt context in the mainline, will be suitable
for that in -rt kernels too.  However, what is suitable for the idle loop
in the mainline, may not be suitable for that in -rt kernels.

That's why raw_spin_lock/unlock() need to be used within the idle loop.

Thanks,
Rafael