Re: [PATCH v2 1/1] ACPI: CPPC: Disable FIE if registers in PCC regions

[Jeremy Linton <jeremy.linton@arm.com>]

Hi,

On 7/29/22 07:59, Punit Agrawal wrote:
> Hi Jeremy,
> 
> One comment / query below.
> 
> Jeremy Linton <jeremy.linton@arm.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@arm.com>
>> ---
>>   drivers/acpi/cppc_acpi.c       | 41 ++++++++++++++++++++++++++++++++++
>>   drivers/cpufreq/cppc_cpufreq.c | 19 ++++++++++++----
>>   include/acpi/cppc_acpi.h       |  5 +++++
>>   3 files changed, 61 insertions(+), 4 deletions(-)
>>
> 
> [...]
> 
>> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
>> index 24eaf0ec344d..ed607e27d6bb 100644
>> --- a/drivers/cpufreq/cppc_cpufreq.c
>> +++ b/drivers/cpufreq/cppc_cpufreq.c
> 
> [...]
> 
>> @@ -229,7 +233,12 @@ static void __init cppc_freq_invariance_init(void)
>>   	};
>>   	int ret;
>>   
>> -	if (cppc_cpufreq_driver.get == hisi_cppc_cpufreq_get_rate)
>> +	if (cppc_perf_ctrs_in_pcc()) {
>> +		pr_debug("FIE not enabled on systems with registers in PCC\n");
> 
> The message should probably be promoted to a pr_info() and exposed as
> part of the kernel logs. It is a change in the default behaviour we've
> had until now. The message will provide some hint about why it was
> disabled.
> 
> Thoughts?

I personally flip flopped between making it pr_info or pr_debug and 
settled on debug because no one else was complaining about the cppc_fie 
consumption. Which to me, meant that the users of platforms utilizing 
PCC regions weren't sensitive to the problem, or weren't yet running a 
distro/kernel with it enabled, or any number of other reasons why the 
problem wasn't getting more attention. Mostly I concluded the FIE code 
hadn't shown up in "enterprise" distros yet..


But, yah, if no one is going to complain about the extra messages 
pr_info() is a better plan.

Thanks for looking at this!