RE: [PATCH V4] cpufreq: intel_pstate: allow trace in passive mode

[Doug Smythies]

On 2018.01.05 14:52 Rafael J. Wysocki wrote:
> On Fri, Jan 5, 2018 at 11:14 PM, Doug Smythies <doug.smythies@xxxxxxxxx> wrote:

>> Allow use of the trace_pstate_sample trace function
>> when the intel_pstate driver is in passive mode.
>> Since the core_busy and scaled_busy fields are not
>> used, and it might be desirable to know which path
>> through the driver was used, either intel_cpufreq_target
>> or intel_cpufreq_fast_switch, re-task the core_busy
>> field as a flag indicator.
>>
>> The user can then use the intel_pstate_tracer.py utility
>> to summarize and plot the trace.
>>
>> Sometimes, in passive mode, the driver is not called for
>> many tens or even hundreds of seconds. The user
>> needs to understand, and not be confused by, this limitation.
>
> The description of the changes between different versions should go
> under the Signed-off-by: tag, separated by an extra "---" from it.

O.K. sorry.

> Also please see a couple of cosmetic comments below.
>
>> V4: Only execute the trace specific overhead code if trace
>>     is enabled. Suggested by Srinivas Pandruvada.
>>
>> V3: Move largely duplicate code to a subroutine.
>>     Suggested by Rafael J. Wysocki.
>>
>> V2: prepare for resend. Rebase to current kernel, 4.15-rc3.
>> Signed-off-by: Doug Smythies <dsmythies@xxxxxxxxx>
>> ---
>>  drivers/cpufreq/intel_pstate.c | 35 +++++++++++++++++++++++++++++++++--
>>  1 file changed, 33 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
>> index 93a0e88..53bb953 100644
>> --- a/drivers/cpufreq/intel_pstate.c
>> +++ b/drivers/cpufreq/intel_pstate.c
>> @@ -1943,13 +1943,40 @@ static int intel_cpufreq_verify_policy(struct cpufreq_policy *policy)
>>         return 0;
>>  }
>>
>> +static void intel_cpufreq_trace(struct cpudata *cpu, int fast, int from)
>
> Please use "bool" for "fast" and I'd call it "fast_switch".

O.K. thanks.

>> +{
>> +       struct sample *sample;
>> +       u64 time;
>> +
>> +       time = ktime_get();
>
> It is pointless to evaluate ktime_get() if
> trace_pstate_sample_enabled() returns "false".

Of course, thanks.

>> +       if (trace_pstate_sample_enabled()) {
>> +               if (intel_pstate_sample(cpu, time)) {
>
> And the extra indentation here is not very useful, so I'd write it as
>
> if (!trace_pstate_sample_enabled())
>        return;
>
> if (!intel_pstate_sample(cpu, ktime_get()))
>        return;
>
> (note that you don't need the "time" variable any more with this).

That is much better, Thanks.

>> +                       sample = &cpu->sample;
>> +                       /* In passvie mode the trace core_busy field is
>
> "passive" (typo)
>
>> +                        * re-assigned to indicate if the driver call
>> +                        * was via the normal or fast switch path.
>> +                        * The scaled_busy field is not used, set to 0.
>> +                        */
>> +                       trace_pstate_sample(fast,
>> +                               0,
>> +                               from,
>> +                               cpu->pstate.current_pstate,
>> +                               sample->mperf,
>> +                               sample->aperf,
>> +                               sample->tsc,
>> +                               get_avg_frequency(cpu),
>> +                               fp_toint(cpu->iowait_boost * 100));
>> +               }
>> +       }
>> +}
>> +
>>  static int intel_cpufreq_target(struct cpufreq_policy *policy,
>>                                 unsigned int target_freq,
>>                                 unsigned int relation)
>>  {
>>         struct cpudata *cpu = all_cpu_data[policy->cpu];
>>         struct cpufreq_freqs freqs;
>> -       int target_pstate;
>> +       int target_pstate, from;
>
> I would call the new variable "old_pstate" or "orig_pstate" (so that
> it is visibly clear that it represents a P-state).

O.K.
I used "from" because that is what Dirk called it in the trace buffer stuff. 

>>
>>         update_turbo_state();
>>
>> @@ -1969,12 +1996,14 @@ static int intel_cpufreq_target(struct cpufreq_policy *policy,
>>                 break;
>>         }
>>         target_pstate = intel_pstate_prepare_request(cpu, target_pstate);
>> +       from = cpu->pstate.current_pstate;
>>         if (target_pstate != cpu->pstate.current_pstate) {
>>                 cpu->pstate.current_pstate = target_pstate;
>>                 wrmsrl_on_cpu(policy->cpu, MSR_IA32_PERF_CTL,
>>                               pstate_funcs.get_val(cpu, target_pstate));
>>         }
>>         freqs.new = target_pstate * cpu->pstate.scaling;
>> +       intel_cpufreq_trace(cpu, 0, from);
>>         cpufreq_freq_transition_end(policy, &freqs, false);
>>
>>         return 0;
>> @@ -1984,13 +2013,15 @@ static unsigned int intel_cpufreq_fast_switch(struct cpufreq_policy *policy,
>>                                               unsigned int target_freq)
>>  {
>>         struct cpudata *cpu = all_cpu_data[policy->cpu];
>> -       int target_pstate;
>> +       int target_pstate, from;
>>
>>         update_turbo_state();
>>
>>         target_pstate = DIV_ROUND_UP(target_freq, cpu->pstate.scaling);
>>         target_pstate = intel_pstate_prepare_request(cpu, target_pstate);
>> +       from = cpu->pstate.current_pstate;
>>         intel_pstate_update_pstate(cpu, target_pstate);
>> +       intel_cpufreq_trace(cpu, 100, from);
>
> Why are you passing 100 here?  Anything different from 0 should
> suffice, 1 in particular.  And I'd pass "false" or "true" (they will
> be converted to 0 and 1 for output anyway).

Well, I wanted to just re-use the existing graphs generated by
tools/power/x86/intel_pstate_tracer/intel_pstate_tracer.py
and so wanted to pass 0 or 100% to it. On purpose, those graphs
do not autoscale on the y-axis.

When investigating, the graphs can be used as a way to determine
where to look in more detail at the raw csv files.

>>         return target_pstate * cpu->pstate.scaling;
>>  }
>>