Re: [PATCH] intel_pstate: track and export frequency residency stats via sysfs.

[Anup Chenthamarakshan]

On Mon, Sep 8, 2014 at 10:03 PM, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> On 9 September 2014 05:40, Anup Chenthamarakshan <anupc@xxxxxxxxxxxx> wrote:
>> Exported stats appear in
>> <sysfs>/devices/system/cpu/intel_pstate/time_in_state as follows:
>>
>> ## CPU 0
>> 400000 3647
>> 500000 24342
>> 600000 144150
>> 700000 202469
>> ## CPU 1
>> 400000 4813
>> 500000 22628
>> 600000 149564
>> 700000 211885
>> 800000 173890
>>
>> Signed-off-by: Anup Chenthamarakshan <anupc@xxxxxxxxxxxx>
>> ---
>>  drivers/cpufreq/intel_pstate.c | 77 ++++++++++++++++++++++++++++++++++++++++--
>>  1 file changed, 74 insertions(+), 3 deletions(-)
>
> Why don't we reuse cpufreq_stats.c for all this?

Thanks for taking a look into this.

I had initially tried reusing cpufreq_stats.c to export stats.
Calling cpufreq_stats_update() via the cpufreq notifier added
some amount of overhead while switching frequencies. Specifically,
looking up the index of the new frequency in freq_table_get_index()
is a linear search through all available frequencies (vs a single
subtraction with custom stats export). Also, the notifier mechanism
itself added a level of indirection before calling stats_update.

There is a 5X increase in time taken to complete intel_pstate_set_pstate
while using cpufreq_stats compared to having custom stats exported.
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