On 09/07/2015 04:04 PM, Rafael J. Wysocki wrote:

On Saturday, September 05, 2015 11:39:20 AM Alan Stern wrote:

quoted

On Sat, 5 Sep 2015, Grygorii Strashko wrote:

quoted

On 09/04/2015 09:45 PM, Alan Stern wrote:

quoted

On Fri, 4 Sep 2015, Grygorii Strashko wrote:

quoted

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. 

Not exactly true :(, since spinlock is converted to [rt_] mutex.
Usually, this difference can't be seen because on -RT kernel all or
mostly all HW IRQ handlers will be forced to be threaded.
For the cases, where such automatic conversion is not working,
(like chained irq handlers or HW-handler+Threaded handler) the code
has to be carefully patched to work properly as for non-RT as for -RT.

Also, this triggers some -RT incompatibility issues, like with PM runtime or
CLK framework (http://www.spinics.net/lists/linux-rt-users/msg13653.html).

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.

Indeed. CPU hotplug/CPUIdle is guarded by local_irq_disable()/local_irq_enable().
(example of CPU hotplug RT-issue http://www.spinics.net/lists/arm-kernel/msg438963.html).

I don't want to be the final authority here as my experience with -RT is short.
But, I want to point out on potential issues based on what I've already discovered
and tried to fix.

Thanks & regards,
-grygorii