The coordination of P-state limits used by intel_pstate in the active
mode (ie. by default) is problematic, because it synchronizes all of
the limits (ie. the global ones and the per-policy ones) so as to use
one common pair of P-state limits (min and max) across all CPUs in
the system. The drawbacks of that are as follows:
- If P-states are coordinated in hardware, it is not necessary
to coordinate them in software on top of that, so in that case
all of the above activity is in vain.
- If P-states are not coordinated in hardware, then the processor
is actually capable of setting different P-states for different
CPUs and coordinating them at the software level simply doesn't
allow that capability to be utilized.
- The coordination works in such a way that setting a per-policy
limit (eg. scaling_max_freq) for one CPU causes the common
effective limit to change (and it will affect all of the other
CPUs too), but subsequent reads from the corresponding sysfs
attributes for the other CPUs will return stale values (which
is confusing).
- Reads from the global P-state limit attributes, min_perf_pct and
max_perf_pct, return the effective common values and not the last
values set through these attributes. However, the last values
set through these attributes become hard limits that cannot be
exceeded by writes to scaling_min_freq and scaling_max_freq,
respectively, and they are not exposed, so essentially users
have to remember what they are.
All of that is painful enough to warrant a change of the management
of P-state limits in the active mode.
To that end, redesign the active mode P-state limits management in
intel_pstate in accordance with the following rules:
(1) All CPUs are affected by the global limits (that is, none of
them can be requested to run faster than the global max and
none of them can be requested to run slower than the global
min).
(2) Each individual CPU is affected by its own per-policy limits
(that is, it cannot be requested to run faster than its own
per-policy max and it cannot be requested to run slower than
its own per-policy min).
(3) The global and per-policy limits can be set independently.
Also, the global maximum and minimum P-state limits will be always
expressed as percentages of the maximum supported turbo P-state.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
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