On Wednesday 29 Jan 2020 at 20:37:41 (+0100), Peter Zijlstra wrote:

On Wed, Dec 18, 2019 at 06:26:06PM +0000, Ionela Voinescu wrote:

quoted

To be noted that this patch is a temporary one. It introduces the
interface added by the patches at [1] to allow update of the frequency
invariance scale factor based on counters. If [1] is merged there is
not need for this patch.

For platforms that support counters (x86 - APERF/MPERF, arm64 - AMU
counters) the frequency invariance correction factor can be obtained
using a core counter and a fixed counter to get information on the
performance (frequency based only) obtained in a period of time. This
will more accurately reflect the actual current frequency of the CPU,
compared with the alternative implementation that reflects the request
of a performance level from the OS through the cpufreq framework
(arch_set_freq_scale).

Therefore, introduce an interface - arch_scale_freq_tick, to be
implemented by each architecture and called for each CPU on the tick
to update the scale factor based on the delta in the counter values,
if counter support is present on the CPU.

Either because reading counters is expensive or because reading
counters from remote CPUs is not possible or is expensive, only
update the counter based frequency scale factor on the tick for
now. A tick based update will definitely be necessary either due to
it being the only point of update for certain architectures or in
order to cache the counter values for a particular CPU, if a
further update from that CPU is not possible.

[1]
https://lore.kernel.org/lkml/20191113124654.18122-1-ggherdovich@suse.cz/ (local)

FWIW, those patches just landed in tip/sched/core

Thanks, Peter, I'll drop this one next time around.

Ionela.

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