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

[Pierre Gondois <pierre.gondois@arm.com>]

On 8/10/22 16:37, Lukasz Luba wrote:
> 
> 
> On 8/10/22 15:30, Jeremy Linton wrote:
>> Hi,
>>
>> On 8/10/22 07:29, 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)?
>>
>> I posted that other email before being awake and conflated MHU with AMU
>> (which could potentially expose the values directly). But the CPPC code
>> isn't aware of whether a MHU or some other mailbox is in use. Either
>> way, its hard to imagine a general mailbox with a doorbell/wait for
>> completion handshake will ever be fast enough to consider running at the
>> granularity this code is running at. If there were a case like that, the
>> kernel would have to benchmark it at runtime to differentiate it from
>> something that is talking over a slow link to a slowly responding mgmt
>> processor.
> 
> Exactly, I'm afraid the same, that we would never get such fast
> mailbox-based platform. Newer platforms would just use AMU, so
> completely different code and no one would even bother to test if
> their HW mailbox is fast-enough for this FIE purpose ;)

To add some platform information, the following platforms are using
CPPC through PCC channels (so mailboxes):
- Cavium ThunderX2
- Ampere eMAG
- Ampere Altra

Fwiw, I can confirm the cppc_fie kthread can represent a significant load,
with a utilization between 2% and 30%.

Regards,
Pierre