cpufreq: intel_pstate: Set asymmetric CPU capacity on hybrid systems

cpufreq: intel_pstate: Set asymmetric CPU capacity on hybrid systems

Make intel_pstate use the HWP_HIGHEST_PERF values from
MSR_HWP_CAPABILITIES to set asymmetric CPU capacity information
via the previously introduced arch_set_cpu_capacity() on hybrid
systems without SMT.

Setting asymmetric CPU capacity is generally necessary to allow the
scheduler to compute task sizes in a consistent way across all CPUs
in a system where they differ by capacity.  That, in turn, should help
to improve scheduling decisions.  It is also necessary for the schedutil
cpufreq governor to operate as expected on hybrid systems where tasks
migrate between CPUs of different capacities.

The underlying observation is that intel_pstate already uses
MSR_HWP_CAPABILITIES to get CPU performance information which is
exposed by it via sysfs and CPU performance scaling is based on it.
Thus using this information for setting asymmetric CPU capacity is
consistent with what the driver has been doing already.  Moreover,
HWP_HIGHEST_PERF reflects the maximum capacity of a given CPU including
both the instructions-per-cycle (IPC) factor and the maximum turbo
frequency and the units in which that value is expressed are the same
for all CPUs in the system, so the maximum capacity ratio between two
CPUs can be obtained by computing the ratio of their HWP_HIGHEST_PERF
values.  Of course, in principle that capacity ratio need not be
directly applicable at lower frequencies, so using it for providing the
asymmetric CPU capacity information to the scheduler is a rough
approximation, but it is as good as it gets.  Also, measurements
indicate that this approximation is not too bad in practice.

If the given system is hybrid and non-SMT, the new code disables ITMT
support in the scheduler (because it may get in the way of asymmetric CPU
capacity code in the scheduler that automatically gets enabled by setting
asymmetric CPU capacity) after initializing all online CPUs and finds
the one with the maximum HWP_HIGHEST_PERF value.  Next, it computes the
capacity number for each (online) CPU by dividing the product of its
HWP_HIGHEST_PERF and SCHED_CAPACITY_SCALE by the maximum HWP_HIGHEST_PERF.

When a CPU goes offline, its capacity is reset to SCHED_CAPACITY_SCALE
and if it is the one with the maximum HWP_HIGHEST_PERF value, the
capacity numbers for all of the other online CPUs are recomputed.  This
also takes care of a cleanup during driver operation mode changes.

Analogously, when a new CPU goes online, its capacity number is updated
and if its HWP_HIGHEST_PERF value is greater than the current maximum
one, the capacity numbers for all of the other online CPUs are
recomputed.

The case when the driver is notified of a CPU capacity change, either
through the HWP interrupt or through an ACPI notification, is handled
similarly to the CPU online case above, except that if the target CPU
is the current highest-capacity one and its capacity is reduced, the
capacity numbers for all of the other online CPUs need to be recomputed
either.

If the driver's "no_trubo" sysfs attribute is updated, all of the CPU
capacity information is computed from scratch to reflect the new turbo
status.

Signed-off-by: Rafael J. Wysocki <[email protected]>
Reviewed-by: Ricardo Neri <[email protected]>
Tested-by: Ricardo Neri <[email protected]> # scale invariance
Link: https://patch.msgid.link/[email protected]
[ rjw: Fixed a typo in the changelog ]
[ rjw: Renamed 3 new functions and added a comment ]
Signed-off-by: Rafael J. Wysocki <[email protected]>