On Mon, Nov 21, 2016 at 04:24:24PM +0000, Patrick Bellasi wrote:
On 21-Nov 16:26, Peter Zijlstra wrote:
In any case, worth trying, see what happens.
Are you saying that you would like to see the code which implements a more generic version of the peak_util "filter" on top of PELT?
Not sure about peak_util, I was more thinking of an IIR/PID filter, as per the email thread referenced below. Doesn't make sense to hide that in intel_pstate if it appears to be universally useful etc..
IMO it could be a good exercise now that we agree we want to improve PELT without replacing it.
I think it would make sense to keep it inside sched_cpufreq for now.
For example, a task running 30 [ms] every 100 [ms] is a ~300 util_avg task. With PELT, we get a signal which range between [120,550] with an average of ~300 which is instead completely ignored. By capping the decay we will get:
decay_cap [ms] range average 0 120:550 300 64 140:560 310 32 320:660 430
which means that still the raw PELT signal is wobbling and never provides a consistent response to drive decisions.
Thus, a "predictor" should be something which sample information from PELT to provide a more consistent view, a sort of of low-pass filter on top of the "dynamic metric" which is PELT.
Should not such a "predictor" help on solving some of the issues related to PELT slow ramp-up or fast ramp-down?
I think intel_pstate recently added a local PID filter, I asked at the time if something like that should live in generic code, looks like maybe it should.
That PID filter is not "just" a software implementation of the ACPI's Collaborative Processor Performance Control (CPPC) when HWP hardware is not provided by a certain processor?
I think it was this thread:
http://lkml.kernel.org/r/1572483.RZjvRFdxPx@vostro.rjw.lan
It never really made sense such a filter should live in individual drivers.