PM: sleep: Resume children after resuming the parent

PM: sleep: Resume children after resuming the parent

According to [1], the handling of device suspend and resume, and
particularly the latter, involves unnecessary overhead related to
starting new async work items for devices that cannot make progress
right away because they have to wait for other devices.

To reduce this problem in the resume path, use the observation that
starting the async resume of the children of a device after resuming
the parent is likely to produce less scheduling and memory management
noise than starting it upfront while at the same time it should not
increase the resume duration substantially.

Accordingly, modify the code to start the async resume of the device's
children when the processing of the parent has been completed in each
stage of device resume and only start async resume upfront for devices
without parents.

Also make it check if a given device can be resumed asynchronously
before starting the synchronous resume of it in case it will have to
wait for another that is already resuming asynchronously.

In addition to making the async resume of devices more friendly to
systems with relatively less computing resources, this change is also
preliminary for analogous changes in the suspend path.

On the systems where it has been tested, this change by itself does
not affect the overall system resume duration in a measurable way.

Link: https://lore.kernel.org/linux-pm/20241114220921.2529905-1-saravanak@google.com/
Suggested-by: Saravana Kannan <saravanak@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/22630663.EfDdHjke4D@rjwysocki.net