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

Jeremy Linton
2022-07-29T15:21:19Z

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!