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

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Message-ID: <f54901f7-fdf9-409a-bbd6-6b08db00e125@amd.com>
Date: Wed, 16 Oct 2024 09:57:21 -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 v2 2/2] cpufreq/amd-pstate: Set the initial min_freq to
 lowest_nonlinear_freq

On 10/16/2024 09:46, 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@....com>
> ---
>   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;

This /could/ be a NULL pointer de-reference.  It should have been 
initialized after the "if (!policy)" check.

It's a one line change though to initialize at the right place so I'll 
do some testing on the series though with that manually fixed up and if 
there are no other problems I'll take it.

Reviewed-by: Mario Limonciello <mario.limonciello@....com>
> +
> +	if (!policy)
> +		return -EINVAL;
> +
> +	if (policy_data->min == FREQ_QOS_MIN_DEFAULT_VALUE)
> +		policy_data->min = cpudata->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);
>   	if (ret < 0) {
>   		dev_err(dev, "Failed to add min-freq constraint (%d)\n", ret);
>   		goto free_cpudata1;