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

[Rafael J. Wysocki]

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.

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".

> +{
> +       struct sample *sample;
> +       u64 time;
> +
> +       time = ktime_get();

It is pointless to evaluate ktime_get() if
trace_pstate_sample_enabled() returns "false".

> +       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).

> +                       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).

>
>         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).

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

Thanks,
Rafael