linux-kernel - Re: [PATCH v3 2/2] cpufreq/amd-pstate: Set the initial min_freq to lowest_nonlinear

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Message-ID: <aadf3614-06ed-4803-91ae-06cd19d1317f@amd.com>
Date: Thu, 17 Oct 2024 09:46:24 -0500
From: Mario Limonciello <mario.limonciello@....com>
To: Dhananjay Ugwekar <Dhananjay.Ugwekar@....com>, gautham.shenoy@....com,
 perry.yuan@....com, rafael@...nel.org, viresh.kumar@...aro.org
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] cpufreq/amd-pstate: Set the initial min_freq to
 lowest_nonlinear_freq

On 10/17/2024 00:39, 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 set by cpufreq-core to
> lowest_nonlinear_freq, in the ->verify() callback. Also set the
> cpudata->req[0] to FREQ_QOS_MIN_DEFAULT_VALUE (i.e. 0), so that it also
> gets overriden 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@....com>

Thanks for the fixes, I'll queue this series up.

Reviewed-by: Mario Limonciello <mario.limonciello@....com>
> ---
>   drivers/cpufreq/amd-pstate.c | 21 ++++++++++++++++++++-
>   1 file changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
> index fa16d72d6058..833fc17a39f2 100644
> --- a/drivers/cpufreq/amd-pstate.c
> +++ b/drivers/cpufreq/amd-pstate.c
> @@ -529,8 +529,27 @@ static void amd_pstate_update(struct amd_cpudata *cpudata, u32 min_perf,
>   
>   static int amd_pstate_verify(struct cpufreq_policy_data *policy_data)
>   {
> +	/*
> +	 * Initialize lower frequency limit (i.e.policy->min) with
> +	 * lowest_nonlinear_frequency which is the most energy efficient
> +	 * frequency. Override the initial value set by cpufreq core and
> +	 * amd-pstate qos_requests.
> +	 */
> +	if (policy_data->min == FREQ_QOS_MIN_DEFAULT_VALUE) {
> +		struct cpufreq_policy *policy = cpufreq_cpu_get(policy_data->cpu);
> +		struct amd_cpudata *cpudata;
> +
> +		if (!policy)
> +			return -EINVAL;
> +
> +		cpudata = policy->driver_data;
> +		policy_data->min = cpudata->lowest_nonlinear_freq;
> +		cpufreq_cpu_put(policy);
> +	}
> +
>   	cpufreq_verify_within_cpu_limits(policy_data);
>   	pr_debug("policy_max =%d, policy_min=%d\n", policy_data->max, policy_data->min);
> +
>   	return 0;
>   }
>   
> @@ -996,7 +1015,7 @@ 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);
>   	if (ret < 0) {
>   		dev_err(dev, "Failed to add min-freq constraint (%d)\n", ret);
>   		goto free_cpudata1;