linux-imx

ANDROID: sched: Update max cpu capacity in case of max frequency constraints

Wakeup balancing uses cpu capacity awareness and needs to know the
system-wide maximum cpu capacity.

Patch "sched: Store system-wide maximum cpu capacity in root domain"
finds the system-wide maximum cpu capacity during scheduler domain
hierarchy setup. This is sufficient as long as maximum frequency
invariance is not enabled.

If it is enabled, the system-wide maximum cpu capacity can change
between scheduler domain hierarchy setups due to frequency capping.

The cpu capacity is changed in update_cpu_capacity() which is called in
load balance on the lowest scheduler domain hierarchy level. To be able
to know if a change in cpu capacity for a certain cpu also has an effect
on the system-wide maximum cpu capacity it is normally necessary to
iterate over all cpus. This would be way too costly. That's why this
patch follows a different approach.

The unsigned long max_cpu_capacity value in struct root_domain is
replaced with a struct max_cpu_capacity, containing value (the
max_cpu_capacity) and cpu (the cpu index of the cpu providing the
maximum cpu_capacity).

Changes to the system-wide maximum cpu capacity and the cpu index are
made if:

 1 System-wide maximum cpu capacity < cpu capacity
 2 System-wide maximum cpu capacity > cpu capacity and cpu index == cpu

There are no changes to the system-wide maximum cpu capacity in all
other cases.

Atomic read and write access to the pair (max_cpu_capacity.val,
max_cpu_capacity.cpu) is enforced by max_cpu_capacity.lock.

The access to max_cpu_capacity.val in task_fits_max() is still performed
without taking the max_cpu_capacity.lock.

The code to set max cpu capacity in build_sched_domains() has been
removed because the whole functionality is now provided by
update_cpu_capacity() instead.

This approach can introduce errors temporarily, e.g. in case the cpu
currently providing the max cpu capacity has its cpu capacity lowered
due to frequency capping and calls update_cpu_capacity() before any cpu
which might provide the max cpu now.

Change-Id: I5063befab088fbf49e5d5e484ce0c6ee6165283a
Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>*
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
(- Fixed cherry-pick issues, and conflict with a0fe2cf086ae "sched/fair:
   Tune down misfit NOHZ kicks" which makes use of max_cpu_capacity
 - Squashed "sched/fair: remove printk while schedule is in progress"
   fix from Caesar Wang <wxt@rock-chips.com>)
Signed-off-by: Quentin Perret <quentin.perret@arm.com>