Questions about sleep states on ARM Juno R0

[rajiv.nishtala@bsc.es (nishtala)]

In response to the discussion here 
(https://community.arm.com/thread/9382). I'm continuing my follow-up 
question.

Sudeep here says to read the sysfs files to get usage/time to understand 
the sleep-state residency of the cores/cluster.

To look at the sysfs files, I wrote a version of a watchdog which 
essentially prints out the difference in "usage", if
it changes in the last 0.01s (it is a LARGE interval relative to update 
period of sysfs files).

The questions I have are the following:
- When the system is idle, that is, no user initiated tasks are running; 
I expected the counter for "usage" in CLUSTER_SLEEP_0 to remain 
constant, because you don't exit
until any kernel interrupts occur, and is guaranteed to stay atleast for 
3500us while the "time" in the same category to increase.

However, what I notice is that "usage" ALSO increases, which is 
something I do not understand.

- When I run an application only on small OR big cores, the counter 
"usage" for CPU_SLEEP_0 on the other type of core (big/small) still 
isn't updated.
Am I missing something here?

- Similarly, when I have an application that does only computations on 
both big AND small cores simultaneously, how come the counter, "usage", 
for CLUSTER_SLEEP_0 increases?
Shouldn't it remain constant?

- How come value of "CLUSTER_SLEEP_0" (on a57 cluster) increases when 
there is actually something running on it. How is it computed?

Any help would be highly appreciated.

Best Regards,
Rajiv Nishtala

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