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    SubjectRe: [RFC PATCH 3/4] cpufreq: Add Active Stats calls tracking frequency changes
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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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