linux-kernel - Re: [PATCH v4 1/2] ACPI: CPPC: Disable FIE if registers in PCC regions

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Message-ID: <641496a1-86f9-6d56-c22c-a77b38a4cf7c@arm.com>
Date:   Thu, 8 Sep 2022 13:42:12 -0500
From:   Jeremy Linton <jeremy.linton@....com>
To:     Punit Agrawal <punit.agrawal@...edance.com>
Cc:     linux-acpi@...r.kernel.org, rafael@...nel.org, lenb@...nel.org,
        viresh.kumar@...aro.org, robert.moore@...el.com,
        lukasz.luba@....com, ionela.voinescu@....com,
        pierre.gondois@....com, linux-kernel@...r.kernel.org,
        devel@...ica.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v4 1/2] ACPI: CPPC: Disable FIE if registers in PCC
 regions

Hi,

On 9/8/22 08:59, Punit Agrawal wrote:
> Hi Jeremy,
> 
> I missed the previous version (holidays) but hopefully still in time for
> this one. A query / comment below.
> 
> Jeremy Linton <jeremy.linton@....com> writes:
> 
>> PCC regions utilize a mailbox to set/retrieve register values used by
>> the CPPC code. This is fine as long as the operations are
>> infrequent. With the FIE code enabled though the overhead can range
>> from 2-11% of system CPU overhead (ex: as measured by top) on Arm
>> based machines.
>>
>> So, before enabling FIE assure none of the registers used by
>> cppc_get_perf_ctrs() are in the PCC region. Furthermore lets also
>> enable a module parameter which can also disable it at boot or module
>> reload.
>>
>> Signed-off-by: Jeremy Linton <jeremy.linton@....com>
>> ---
>>   drivers/acpi/cppc_acpi.c       | 41 ++++++++++++++++++++++++++++++++++
>>   drivers/cpufreq/cppc_cpufreq.c | 31 +++++++++++++++++++++----
>>   include/acpi/cppc_acpi.h       |  5 +++++
>>   3 files changed, 73 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
>> index 1e15a9f25ae9..c840bf606b30 100644
>> --- a/drivers/acpi/cppc_acpi.c
>> +++ b/drivers/acpi/cppc_acpi.c
>> @@ -1240,6 +1240,47 @@ int cppc_get_perf_caps(int cpunum, struct cppc_perf_caps *perf_caps)
>>   }
>>   EXPORT_SYMBOL_GPL(cppc_get_perf_caps);
>>   
>> +/**
>> + * cppc_perf_ctrs_in_pcc - Check if any perf counters are in a PCC region.
>> + *
>> + * CPPC has flexibility about how counters describing CPU perf are delivered.
>> + * One of the choices is PCC regions, which can have a high access latency. This
>> + * routine allows callers of cppc_get_perf_ctrs() to know this ahead of time.
>> + *
>> + * Return: true if any of the counters are in PCC regions, false otherwise
>> + */
>> +bool cppc_perf_ctrs_in_pcc(void)
>> +{
>> +	int cpu;
>> +
>> +	for_each_present_cpu(cpu) {
>> +		struct cpc_register_resource *ref_perf_reg;
>> +		struct cpc_desc *cpc_desc;
>> +
>> +		cpc_desc = per_cpu(cpc_desc_ptr, cpu);
>> +
>> +		if (CPC_IN_PCC(&cpc_desc->cpc_regs[DELIVERED_CTR]) ||
>> +		    CPC_IN_PCC(&cpc_desc->cpc_regs[REFERENCE_CTR]) ||
>> +		    CPC_IN_PCC(&cpc_desc->cpc_regs[CTR_WRAP_TIME]))
>> +			return true;
>> +
>> +
>> +		ref_perf_reg = &cpc_desc->cpc_regs[REFERENCE_PERF];
>> +
>> +		/*
>> +		 * If reference perf register is not supported then we should
>> +		 * use the nominal perf value
>> +		 */
>> +		if (!CPC_SUPPORTED(ref_perf_reg))
>> +			ref_perf_reg = &cpc_desc->cpc_regs[NOMINAL_PERF];
>> +
>> +		if (CPC_IN_PCC(ref_perf_reg))
>> +			return true;
>> +	}
>> +	return false;
>> +}
>> +EXPORT_SYMBOL_GPL(cppc_perf_ctrs_in_pcc);
>> +
>>   /**
>>    * cppc_get_perf_ctrs - Read a CPU's performance feedback counters.
>>    * @cpunum: CPU from which to read counters.
>> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
>> index 24eaf0ec344d..32fcb0bf74a4 100644
>> --- a/drivers/cpufreq/cppc_cpufreq.c
>> +++ b/drivers/cpufreq/cppc_cpufreq.c
>> @@ -63,7 +63,15 @@ static struct cppc_workaround_oem_info wa_info[] = {
>>   
>>   static struct cpufreq_driver cppc_cpufreq_driver;
>>   
>> +static enum {
>> +	FIE_UNSET = -1,
>> +	FIE_ENABLED,
>> +	FIE_DISABLED
>> +} fie_disabled = FIE_UNSET;
>> +
>>   #ifdef CONFIG_ACPI_CPPC_CPUFREQ_FIE
>> +module_param(fie_disabled, int, 0444);
>> +MODULE_PARM_DESC(fie_disabled, "Disable Frequency Invariance Engine (FIE)");
>>   
>>   /* Frequency invariance support */
>>   struct cppc_freq_invariance {
>> @@ -158,7 +166,7 @@ static void cppc_cpufreq_cpu_fie_init(struct cpufreq_policy *policy)
>>   	struct cppc_freq_invariance *cppc_fi;
>>   	int cpu, ret;
>>   
>> -	if (cppc_cpufreq_driver.get == hisi_cppc_cpufreq_get_rate)
>> +	if (fie_disabled)
>>   		return;
> 
> With this change, if FIE is enabled, the rest of the function will run
> even if the hisi workaround is enabled. Not sure if that is an
> intentional change. The same applies to similar other changes in the
> patch as well.

Yah, I think its intentional, unless i'm missing something. The hisi 
quirk detection path forces this off regardless of the user attempting 
to force it on. Which is part of why I think the enum states must be as 
above. The other reason is that the final result of whether FIE is 
disabled ends up in /sys/modules/cppc_cpufreq/parameters/fie_disabled 
which in this case may not reflect what the user requested.


I have another patch that might be worth posting that I created while 
implementing CPPC on a machine a year or so ago that removes this quirk 
entirely. Instead it detects counters that aren't incrementing properly 
and NULL's out the get routine so that cpupower/etc report that the 
frequency is being retrieved from the kernel rather than the hardware. 
Someone else will have to test it though, because I eventually figured 
out how to synthesize both counters in a way that is generic enough to 
work on most machines.