Messages in this thread | | | Date | Thu, 29 Jun 2017 09:58:36 +0530 | From | Viresh Kumar <> | Subject | Re: [PATCH] cpufreq: Find transition latency dynamically |
| |
On 02-06-17, 16:59, 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. > > Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> > --- > drivers/cpufreq/cpufreq.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 63 insertions(+)
Hi Rafael,
Any inputs on this one ?
-- viresh
| |