Re: [PATCH v2 2/2] cpufreq/amd-pstate: Set the initial min_freq to lowest_nonlinear_freq

[Gautham R. Shenoy]

Hello Dhananjay,

On Wed, Oct 16, 2024 at 02:46:42PM +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.
> 
> Modify the initial policy_data->min passed by cpufreq core to
> lowest_nonlinear_freq, in the ->verify() callback. Also set the
> qos_request cpudata->req[0] to FREQ_QOS_MIN_DEFAULT_VALUE (i.e. 0), 
> so that it also gets overridden by the check in verify function.
> 
> Link: https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24593.pdf [1]
> 
> Signed-off-by: Dhananjay Ugwekar <Dhananjay.Ugwekar@xxxxxxx>
> ---
>  drivers/cpufreq/amd-pstate.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
> index fa16d72d6058..117ad5988e8e 100644
> --- a/drivers/cpufreq/amd-pstate.c
> +++ b/drivers/cpufreq/amd-pstate.c
> @@ -529,8 +529,20 @@ static void amd_pstate_update(struct amd_cpudata *cpudata, u32 min_perf,
>  
>  static int amd_pstate_verify(struct cpufreq_policy_data *policy_data)
>  {
> +	struct cpufreq_policy *policy = cpufreq_cpu_get(policy_data->cpu);
> +	struct amd_cpudata *cpudata = policy->driver_data;
> +
> +	if (!policy)
> +		return -EINVAL;
> +
> +	if (policy_data->min == FREQ_QOS_MIN_DEFAULT_VALUE)
> +		policy_data->min = cpudata->lowest_nonlinear_freq;

Why not unconditionally set policy->min to lowest_nonlinear_freq ?


> +
>  	cpufreq_verify_within_cpu_limits(policy_data);
>  	pr_debug("policy_max =%d, policy_min=%d\n", policy_data->max, policy_data->min);
> +
> +	cpufreq_cpu_put(policy);
> +
>  	return 0;
>  }
>  
> @@ -996,7 +1008,7 @@ static int amd_pstate_cpu_init(struct cpufreq_policy *policy)
>  		policy->fast_switch_possible = true;
>  
>  	ret = freq_qos_add_request(&policy->constraints, &cpudata->req[0],
> -				   FREQ_QOS_MIN, policy->cpuinfo.min_freq);
> +				   FREQ_QOS_MIN, FREQ_QOS_MIN_DEFAULT_VALUE);


This qos request can still be set to cpuinfo.min_freq, no ? Especially
if you unconditionally initialize policy->min to lowest_nonlinear_freq
in amd_pstate_policy, no?

--
Thanks and Regards
gautham.