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SubjectCPU Hotplug add/remove optimizations
  Hi,

We are trying to use cpu hotplug to turn off a cpu when it is not in use
to improve power management. I am trying to optimize the cpu hotplug add
and cpu hotplug remove timings. Currently cpu hotplug add takes around
250ms and cpu hotplug remove takes 190 ms. For the current purposes we
want to assume that we are removing and adding the same core. It seems
that since we are actually not replacing the core – there could be a lot
of initialization overhead that could be saved and restored instead of
calibrating the entire core again.
One such thing we have been looking at is that once a core is powered up
during cpu hotplug add, it runs the calibrate_delay routine to calculate
the value of loops_per_jiffy. In such a case could we bypass the
calibrate_delay function and just save and restore the value of
loops_per_jiffy?
Does this approach seem wrong to anyone?
Can we safely assume that the core will start at the same clock speed at
which the value was stored and then restored? And thus keeping the value
of loops_per_jiffy that we restored consistent with the rest of the system.
Does anyone else have other ideas to achieve better results?
I will be looking at implementing any solutions and advice and following
up with metrics and results.

Thanks,
Rohit Vaswani

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Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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