Re: [PATCH 2/3] cpufreq/amd-pstate: Set the initial min_freq to lowest_nonlinear_freq

[Gautham R. Shenoy]

On Thu, Oct 03, 2024 at 08:39:54AM +0000, Dhananjay Ugwekar wrote:
> According to the AMD architectural programmer's manual volume 2 [1], in
> section "17.6.4.1 CPPC_CAPABILITY_1" lowest_nonlinear_perf is described
> as "Reports the most energy efficient performance level (in terms of
> performance per watt). Above this threshold, lower performance levels
> generally result in increased energy efficiency. Reducing performance
> below this threshold does not result in total energy savings for a given
> computation, although it reduces instantaneous power consumption". So
> lowest_nonlinear_perf is the most power efficient performance level, and
> going below that would lead to a worse performance/watt.
> 
> Also, setting the minimum frequency to lowest_nonlinear_freq (instead of
> lowest_freq) allows the CPU to idle at a higher frequency which leads
> to more time being spent in a deeper idle state (as trivial idle tasks
> are completed sooner). This has shown a power benefit in some systems,
> in other systems, power consumption has increased but so has the
> throughput/watt.
> 
> Use the get_init_min_freq() callback to set the initial lower limit for
> amd-pstate driver to lowest_nonlinear_freq instead of lowest_freq.
> 
> Link: https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24593.pdf [1]

It is good enough to quote "AMD64 Architecture Programmer's Manual,
Volume 2, Revision 3.42" instead of providing a link.

Other than that,

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@xxxxxxx>



> 
> Signed-off-by: Dhananjay Ugwekar <Dhananjay.Ugwekar@xxxxxxx>


--
Thanks and Regards
gautham.

> ---
>  drivers/cpufreq/amd-pstate.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
> index b7a17a3ef122..f8abae9a0156 100644
> --- a/drivers/cpufreq/amd-pstate.c
> +++ b/drivers/cpufreq/amd-pstate.c
> @@ -995,13 +995,6 @@ static int amd_pstate_cpu_init(struct cpufreq_policy *policy)
>  	if (cpu_feature_enabled(X86_FEATURE_CPPC))
>  		policy->fast_switch_possible = true;
>  
> -	ret = freq_qos_add_request(&policy->constraints, &cpudata->req[0],
> -				   FREQ_QOS_MIN, policy->cpuinfo.min_freq);
> -	if (ret < 0) {
> -		dev_err(dev, "Failed to add min-freq constraint (%d)\n", ret);
> -		goto free_cpudata1;
> -	}
> -
>  	ret = freq_qos_add_request(&policy->constraints, &cpudata->req[1],
>  				   FREQ_QOS_MAX, policy->cpuinfo.max_freq);
>  	if (ret < 0) {
> @@ -1706,6 +1699,13 @@ static int amd_pstate_epp_resume(struct cpufreq_policy *policy)
>  	return 0;
>  }
>  
> +static int amd_pstate_get_init_min_freq(struct cpufreq_policy *policy)
> +{
> +	struct amd_cpudata *cpudata = policy->driver_data;
> +
> +	return READ_ONCE(cpudata->lowest_nonlinear_freq);
> +}
> +
>  static struct cpufreq_driver amd_pstate_driver = {
>  	.flags		= CPUFREQ_CONST_LOOPS | CPUFREQ_NEED_UPDATE_LIMITS,
>  	.verify		= amd_pstate_verify,
> @@ -1719,6 +1719,7 @@ static struct cpufreq_driver amd_pstate_driver = {
>  	.update_limits	= amd_pstate_update_limits,
>  	.name		= "amd-pstate",
>  	.attr		= amd_pstate_attr,
> +	.get_init_min_freq = amd_pstate_get_init_min_freq,
>  };
>  
>  static struct cpufreq_driver amd_pstate_epp_driver = {
> @@ -1735,6 +1736,7 @@ static struct cpufreq_driver amd_pstate_epp_driver = {
>  	.set_boost	= amd_pstate_set_boost,
>  	.name		= "amd-pstate-epp",
>  	.attr		= amd_pstate_epp_attr,
> +	.get_init_min_freq = amd_pstate_get_init_min_freq,
>  };
>  
>  static int __init amd_pstate_set_driver(int mode_idx)
> -- 
> 2.34.1
>