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

Lukasz Luba
2022-08-10T13:56:53Z

On 8/10/22 13:51, Ionela Voinescu wrote:
> Hi folks,
>
> On Wednesday 10 Aug 2022 at 13:29:08 (+0100), Lukasz Luba wrote:
>> Hi Jeremy,
>>
>> +CC Valentin since he might be interested in this finding
>> +CC Ionela, Dietmar
>>
>> I have a few comments for this patch.
>>
>>
>> On 7/28/22 23:10, Jeremy Linton wrote:
>>> 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(-)
>>
>>
>> 1. You assume that all platforms would have this big overhead when
>> they have the PCC regions for this purpose.
>> Do we know which version of HW mailbox have been implemented
>> and used that have this 2-11% overhead in a platform?
>> Do also more recent MHU have such issues, so we could block
>> them by default (like in your code)?
>>
>> 2. I would prefer to simply change the default Kconfig value to 'n' for
>> the ACPI_CPPC_CPUFREQ_FIE, instead of creating a runtime
>> check code which disables it.
>> We have probably introduce this overhead for older platforms with
>> this commit:
>>
>> commit 4c38f2df71c8e33c0b64865992d693f5022eeaad
>> Author: Viresh Kumar <viresh.kumar@linaro.org>
>> Date: Tue Jun 23 15:49:40 2020 +0530
>>
>> cpufreq: CPPC: Add support for frequency invariance
>>
>>
>>
>> If the test server with this config enabled performs well
>> in the stress-tests, then on production server the config may be
>> set to 'y' (or 'm' and loaded).
>>
>> I would vote to not add extra code, which then after a while might be
>> decided to bw extended because actually some HW is actually capable (so
>> we could check in runtime and enable it). IMO this create an additional
>> complexity in our diverse configuration/tunnable space in our code.
>>
>
> I agree that having CONFIG_ACPI_CPPC_CPUFREQ_FIE default to no is the
> simpler solution but it puts the decision in the hands of platform
> providers which might result in this functionality not being used most
> of the times, if at all. This being said, the use of CPPC counters is
> meant as a last resort for FIE, if the platform does not have AMUs. This
> is why I recommended this to default to no in the review of the original
> patches.
>
> But I don't see these runtime options as adding a lot of complexity
> and therefore agree with the idea of this patch, versus the config
> change above, with two design comments:
> - Rather than having a check for fie_disabled in multiple init and exit
> functions I think the code should be slightly redesigned to elegantly
> bail out of most functions if cppc_freq_invariance_init() failed.
> - Given the multiple options to disable this functionality (config,
> PCC check), I don't see a need for a module parameter or runtime user
> input, unless we make that overwrite all previous decisions, as in: if
> CONFIG_ACPI_CPPC_CPUFREQ_FIE=y, even if cppc_perf_ctrs_in_pcc(), if
> the fie_disabled module parameter is no, then counters should be used
> for FIE.
>

A few things:
1. With this default CONFIG_ACPI_CPPC_CPUFREQ_FIE=y we've introduced
a performance regression on older HW servers, which is not good IMO.
It looks like it wasn't a good idea. The FIE which is used in a tick
and going through mailbox and FW sounds like a bad design.
You need to have a really fast HW mailbox, FW and uC running it,
to be able to provide a decent performance.
2. Keeping a code which is not used in a server because at runtime we
discover this PCC overhead issue doesn't make sense.
3. System integrator or distro engineers should be able to experiment
with different kernel config options on the platform and disable/
enable this option on particular server. I am afraid that we cannot
figure out and assume performance at runtime in this code and say
it would be good or not to use it. Only stress-tests would tell this.