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

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Date:   Wed, 10 Aug 2022 15:37:54 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     rafael@...nel.org, lenb@...nel.org, viresh.kumar@...aro.org,
        robert.moore@...el.com, devel@...ica.org,
        linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, vschneid@...hat.com,
        Ionela Voinescu <ionela.voinescu@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v2 1/1] ACPI: CPPC: Disable FIE if registers in PCC
 regions



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@....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 ;)