Messages in this thread | | | Subject | Re: [PATCH] cpufreq: Find transition latency dynamically | From | Leonard Crestez <> | Date | Tue, 6 Jun 2017 18:48:30 +0300 |
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On Fri, 2017-06-02 at 16:59 +0530, Viresh Kumar wrote: > The transition_latency_ns represents the maximum time it can take for > the hardware to switch from/to any frequency for a CPU. > > The transition_latency_ns is used currently for two purposes: > > o To check if the hardware latency is over the maximum allowed for a > governor (only for ondemand and conservative (why not schedutil?)) and > to decide if the governor can be used or not. > > o To calculate the sampling_rate or rate_limit for the governors by > multiplying transition_latency_ns with a constant. > > The platform drivers can also set this value to CPUFREQ_ETERNAL if they > don't know this number and in that case we disallow use of ondemand and > conservative governors as the latency would be higher than the maximum > allowed for the governors. > > In many cases this number is forged by the driver authors to get the > default sampling rate to a desired value. Anyway, the actual latency > values can differ from what is received from the hardware designers. > > Over that, what is provided by the drivers is most likely the time it > takes to change frequency of the hardware, which doesn't account the > software overhead involved. > > In order to have guarantees about this number, this patch tries to > calculate the latency dynamically at cpufreq driver registration time by > first switching to min frequency, then to the max and finally back to > the initial frequency. And the maximum of all three is used as the > target_latency. Specifically the time it takes to go from min to max > frequency (when the software runs the slowest) should be good enough, > and even if there is a delta involved then it shouldn't be a lot. > > For now this patch limits this feature only for platforms which have set > the transition latency to CPUFREQ_ETERNAL. Maybe we can convert everyone > to use it in future, but lets see. > > This is tested over ARM64 Hikey platform which currently sets > "clock-latency" as 500 us from DT, while with this patch the actualy > value increased to 800 us.
I remember checking if transition latency is correct for imx6q-cpufreq and it does not appear to be. Maybe because i2c latency of regulator adjustments is not counted in?
It seems to me it would be much nicer to have a special flag for this instead of overriding CPUFREQ_ETERNAL.
Also, wouldn't it be possible to update this dynamically? Just measure the duration every time it happens and do an update like latency = (latency * 7 + latest_latency) / 8.
-- Regards, Leonard
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