Optimize cpuidle module initialization by using the new Target.grep_values
call to get information about all idle states in a single call to the
target during module intialization, rather lazily fetching them from the
target afterwards.
Unit info from comments on `struct cpuidle_state` definition in
include/linux/cpuidle.h in Linux 4.9 source (see drivers/cpuidle/sysfs.c
for the code that exposes that data to userspace)
On some systems CPUs sometimes remain idle for several seconds. If a
trace capture begins during one of these long idle periods, that CPU's
idle state is unknown (in practice it is probably in its deepest
available state from cpuidle's perspective but that can't be known for
sure).
The solution to the equivalent problem for cpufreq is to read the
current frequencies from sysfs and inject artificial cpu_frequency
events using trace_marker (see cpu_freq_trace_all_frequencies in
bin/scripts/shutils.in). However we can't read the current idle state
from sysfs.
Instead, wake up each CPU by executing the "true" command on it via
taskset.
When this information is not needed, this avoids executing 4 commands on
the host for each CPU, which significantly speeds up initialising the
cpuidle module.
When passing an idle state ID to get_states(), nothing is returned. Also, there
is a wrong call to enable() in the method.
Signed-off-by: Michele Di Giorgio <michele.digiorgio@arm.com>
