linux-kernel - Re: [RFC/RFT] [PATCH 02/10] cpufreq: intel_pstate: Conditional frequency invariant accounting

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Date:   Wed, 16 May 2018 17:19:25 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, peterz@...radead.org,
        bp@...e.de, lenb@...nel.org, rjw@...ysocki.net,
        mgorman@...hsingularity.net, x86@...nel.org,
        linux-pm@...r.kernel.org, viresh.kumar@...aro.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC/RFT] [PATCH 02/10] cpufreq: intel_pstate: Conditional
 frequency invariant accounting

On 15/05/18 21:49, Srinivas Pandruvada wrote:
> intel_pstate has two operating modes: active and passive. In "active"
> mode, the in-built scaling governor is used and in "passive" mode,
> the driver can be used with any governor like "schedutil". In "active"
> mode the utilization values from schedutil is not used and there is
> a requirement from high performance computing use cases, not to read
> any APERF/MPERF MSRs. In this case no need to use CPU cycles for
> frequency invariant accounting by reading APERF/MPERF MSRs.
> With this change frequency invariant account is only enabled in
> "passive" mode.
> 
> Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> ---
> [Note: The tick will be enabled later in the series when hwp dynamic
> boost is enabled]
> 
>  drivers/cpufreq/intel_pstate.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index 17e566af..f686bbe 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -2040,6 +2040,8 @@ static int intel_pstate_register_driver(struct cpufreq_driver *driver)
>  {
>  	int ret;
>  
> +	x86_arch_scale_freq_tick_disable();
> +
>  	memset(&global, 0, sizeof(global));
>  	global.max_perf_pct = 100;
>  
> @@ -2052,6 +2054,9 @@ static int intel_pstate_register_driver(struct cpufreq_driver *driver)
>  
>  	global.min_perf_pct = min_perf_pct_min();
>  
> +	if (driver == &intel_cpufreq)
> +		x86_arch_scale_freq_tick_enable();

This will unconditionally trigger the reading/calculation at each tick
even though information is not actually consumed (e.g., running
performance or any other governor), right? Do we want that?

Anyway, FWIW I started testing this on a E5-2609 v3 and I'm not seeing
hackbench regressions so far (running with schedutil governor).

Best,

- Juri