Messages in this thread | | | Subject | Re: [RFC PATCH 3/4] cpufreq: Add Active Stats calls tracking frequency changes | From | Lukasz Luba <> | Date | Tue, 22 Jun 2021 16:10:07 +0100 |
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On 6/22/21 3:59 PM, Rafael J. Wysocki wrote: > On Tue, Jun 22, 2021 at 4:51 PM Rafael J. Wysocki <rafael@kernel.org> wrote: >> >> On Tue, Jun 22, 2021 at 4:09 PM Lukasz Luba <lukasz.luba@arm.com> wrote: >>> >>> >>> >>> On 6/22/21 2:51 PM, Rafael J. Wysocki wrote: >>>> On Tue, Jun 22, 2021 at 3:42 PM Lukasz Luba <lukasz.luba@arm.com> wrote: >>>>> >>>>> >>>>> >>>>> On 6/22/21 1:28 PM, Rafael J. Wysocki wrote: >>>>>> On Tue, Jun 22, 2021 at 9:59 AM Lukasz Luba <lukasz.luba@arm.com> wrote: >>>>>>> >>>>>>> The Active Stats framework tracks and accounts the activity of the CPU >>>>>>> for each performance level. It accounts the real residency, when the CPU >>>>>>> was not idle, at a given performance level. This patch adds needed calls >>>>>>> which provide the CPU frequency transition events to the Active Stats >>>>>>> framework. >>>>>>> >>>>>>> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com> >>>>>>> --- >>>>>>> drivers/cpufreq/cpufreq.c | 5 +++++ >>>>>>> 1 file changed, 5 insertions(+) >>>>>>> >>>>>>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c >>>>>>> index 802abc925b2a..d79cb9310572 100644 >>>>>>> --- a/drivers/cpufreq/cpufreq.c >>>>>>> +++ b/drivers/cpufreq/cpufreq.c >>>>>>> @@ -14,6 +14,7 @@ >>>>>>> >>>>>>> #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt >>>>>>> >>>>>>> +#include <linux/active_stats.h> >>>>>>> #include <linux/cpu.h> >>>>>>> #include <linux/cpufreq.h> >>>>>>> #include <linux/cpu_cooling.h> >>>>>>> @@ -387,6 +388,8 @@ static void cpufreq_notify_transition(struct cpufreq_policy *policy, >>>>>>> >>>>>>> cpufreq_stats_record_transition(policy, freqs->new); >>>>>>> policy->cur = freqs->new; >>>>>>> + >>>>>>> + active_stats_cpu_freq_change(policy->cpu, freqs->new); >>>>>>> } >>>>>>> } >>>>>>> >>>>>>> @@ -2085,6 +2088,8 @@ unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy, >>>>>>> policy->cpuinfo.max_freq); >>>>>>> cpufreq_stats_record_transition(policy, freq); >>>>>>> >>>>>>> + active_stats_cpu_freq_fast_change(policy->cpu, freq); >>>>>>> + >>>>>> >>>>>> This is quite a bit of overhead and so why is it needed in addition to >>>>>> the code below? >>>>> >>>>> The code below is tracing, which is good for post-processing. We use in >>>>> our tool LISA, when we analyze the EAS decision, based on captured >>>>> trace data. >>>>> >>>>> This new code is present at run time, so subsystems like our thermal >>>>> governor IPA can use it and get better estimation about CPU used power >>>>> for any arbitrary period, e.g. 50ms, 100ms, 300ms, ... >>>> >>>> So can it be made not run when the IPA is not using it? >>> >>> I can make a Kconfig for IPA to select this ACTIVE_STATS. >>> Also, I can add description that this framework is mostly needed >>> for IPA, so don't enable it if you don't use IPA (default is 'n' >>> so it shouldn't harm others). >>> >>> This Active Stats shouldn't be stopped when thermal zone is switching >>> between governors at run time, e.g. IPA -> step_wise -> IPA >>> because when IPA is set next time, it might not have correct CPU >>> stats (what is the current frequency and for how long it has been >>> actively used). >> >> But after a while it will collect enough useful data I suppose? >> >>> Beside, switching governors at run time is not a good idea >>> (apart from stress testing them ;) ). >>> >>>> >>>>>> >>>>>> And pretty much the same goes for the idle loop change. There is >>>>>> quite a bit of instrumentation in that code already and it avoids >>>>>> adding new locking for a reason. Why is it a good idea to add more >>>>>> locking to that code? >>>>> >>>>> This active_stats_cpu_freq_fast_change() doesn't use the locking, it >>>>> relies on schedutil lock in [1]. >>>> >>>> Ah, OK. >>>> >>>> But it still adds overhead AFAICS. >>> >>> Agree, it's an extra code. For platforms which use IPA it's a >>> justifiable cost, weighted by better estimation thanks to this calls. >>> For other platforms, this framework will be set to default 'n' option. >> >> A general problem with build-time configuration is for distros that >> want to ship one kernel binary to run on multiple hardware platforms. >> They need to enable those options anyway and then get the full cost on >> the platforms that don't need it, but want to use the common binary >> kernel. >> >> Again, please consider making this new code run only when it is needed >> even if configured in and if it runs, make it as low-overhead as >> possible. > > Also, why don't you add these hooks to the drivers that are generally > worked with by the IPA?
In Arm world (especially 32-bit world) there is 'a lot' custom idle and cpufreq drivers. It's probably even not feasible to do. We also has this CPU_IDLE_MULTIPLE_DRIVERS mechanism. It's a pain, especially when not having all possible platfroms.
> > That you won't need to worry about the possible impact on everybody else. >
I'll try to make it as low-overhead as possible and turn off if there is no client subsystem (like IPA) currently using it. That might be feasible.
Thank you Rafael for valuable comments.
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