Re: [PATCH 0/2] CPPC: reduce FFH feedback-counter sampling skew on arm64

[Pengjie Zhang <zhangpengjie2@huawei.com>]

On 6/26/2026 10:55 PM, Vanshidhar Konda wrote:
> On Wed, May 20, 2026 at 10:55:54AM +0800, Pengjie Zhang wrote:
>>
>> On 5/19/2026 6:47 PM, Will Deacon wrote:
>>> On Thu, Apr 30, 2026 at 06:00:44PM +0800, zhangpengjie (A) wrote:
>>>> Hi all,
>>>>
>>>> Gentle ping on this thread. It has been a while since I posted it.
>>>>
>>>> Could someone please take a look when you have time? If there is 
>>>> anything
>>>> I should revise or any additional information needed, I'd be happy to
>>>> update it.
>>> It's hard to find active folks who have contributed meaningfully to the
>>> cppc_acpi driver... I've added Ionella and Jeremy, in case they can 
>>> take
>>> a look.
>>>
>>> Will
>> Thanks Will, and thanks for adding Ionela and Jeremy.
>>
>> While waiting for further comments, I would like to add some
>> test data to make the effect of this series clearer.
>>
>> On the test platform, the maximum frequency reported by the platform
>> is 2300000. I sampled cpuinfo_cur_freq before and after applying this 
>> series.
>>
>> Before applying the series, the samples showed visible transient 
>> outliers.
>> ??The minimum value was 2154583 and the maximum value was 2491071.
>> There were 8 samples above 2400000 and 8 samples below 2200000.
>> The largest value exceeded the platform maximum by about 8.3%.
>>
>> After applying the series, the samples became much more stable.
>> The minimum value was 2290243 and the maximum value was 2306310.
>> There were no samples above 2400000 and no samples below 2200000.
>> The largest value exceeded the platform maximum by only about 0.27%.
>>
>> A summary of the 96 samples is:
>>
>> ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? before?? ?? ?? ?? ?? after
>> min?? ?? ?? ?? ?? ?? ?? 2154583?? ?? ?? ?? ??2290243
>> max?? ?? ?? ?? ?? ?? ?? 2491071?? ?? ?? ?? ??2306310
>> range?? ?? ?? ?? ?? ?? ??336488?? ?? ?? ?? ?? ??16067
>> average?? ?? ?? ?? ?? 2298436.4?? ?? ?? ??2298479.4
>> stddev?? ?? ?? ?? ?? ?? ??55184.1?? ?? ?? ?? ?? 2868.2
>> samples > 2300000?? 26 / 96?? ?? ?? ?? ??16 / 96
>> samples > 2400000?? ??8 / 96?? ?? ?? ?? ?? 0 / 96
>> samples < 2200000?? ??8 / 96?? ?? ?? ?? ?? 0 / 96
>>
>> So this series does not try to clamp the value to the platform maximum.
>> ??Instead, it reduces the sampling skew between the delivered and 
>> reference
>> feedback counters. The remaining small deviation around 2300000 is
>> ??much smaller than the previous transient spikes.
>>
>> One concern that may come up is that an FFH read may cause an idle
>> target CPU to be woken, depending on the platform/vendor implementation.
>> However, that behavior is not introduced by this series. It is 
>> already part
>> of how FFH counter reads are implemented on such platforms. This series
>> only changes the sampling form for the FFH feedback counters: when both
>> delivered and reference counters are FFH counters, read them together
>> instead of issuing two separate FFH reads.
>>
>> If the target CPU has to be involved for an FFH read, doing one paired
>> read should be no worse than doing two separate reads, and it also
>> narrows the sampling window between the two counters.
>>
>
> I agree with this point. It reduces the number of times the idle CPU
> is woken up just to read counters.
>
>> If there is any concern about the generic hook or the arm64 
>> implementation,
>> I would be happy to revise it.
>>
>> The raw data is as follows:
>> before:
>> 2303809 2294827 2300000 2293643 2290740 2300000 2297228 2296082
>> 2301707 2295354 2296601 2303163 2296766 2296543 2295412 2298394
>> 2297387 2300000 2308274 2301882 2297752 2418568 2491071 2300000
>> 2183264 2296238 2434731 2296721 2439777 2302159 2301773 2298226
>> 2300000 2305936 2301133 2297511 2300000 2300000 2294408 2298494
>> 2295011 2302721 2295955 2301505 2298064 2297419 2298933 2189595
>> 2298058 2296046 2300000 2301449 2414908 2296559 2305251 2166666
>> 2296626 2173303 2300000 2298806 2411389 2301822 2297291 2300000
>> 2423831 2297902 2300000 2435730 2302433 2295353 2298898 2296043
>> 2321868 2294907 2300000 2157841 2296052 2206530 2300000 2297811
>> 2297920 2294382 2297767 2157230 2302564 2298504 2296822 2300000
>> 2296868 2294866 2154583 2290888 2302542 2292549 2300000 2184259
>>
>> after:
>> 2303738 2296153 2298087 2295607 2301373 2298076 2300000 2295081
>> 2297788 2300000 2300000 2295238 2301449 2300000 2298488 2297911
>> 2301477 2298507 2294976 2296852 2293689 2294077 2293887 2292619
>> 2300000 2300000 2298072 2300000 2291943 2300000 2295370 2300000
>> 2301873 2304645 2300000 2296766 2300000 2300000 2290243 2297954
>> 2297183 2306310 2300000 2296889 2300000 2303800 2301970 2296888
>> 2300000 2301354 2300000 2298405 2298202 2296767 2298663 2302522
>> 2297821 2302471 2300000 2303233 2298226 2298698 2300000 2297291
>> 2296470 2300000 2298398 2300000 2295681 2300000 2300000 2296344
>> 2300000 2296008 2302375 2297977 2298447 2296519 2295565 2294866
>> 2297945 2300000 2294978 2303595 2300000 2300000 2294930 2301096
>> 2296271 2296086 2294482 2300000 2294843 2300000 2296803 2295708
>
> I tested this series on Ampere AmpereOne A192-32X.
>
> Tested-by: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
> Reviewed-by: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
>
> Regards,
> Vanshidhar Konda
>
Hi Vanshidhar,

Thank you for taking the time to review and test this series, especially
on the AmpereOne A192-32X platform. It's great to know it works well there.
I will add your tags to the next version.

Thanks again,
     Pengjie


>>>> On 4/10/2026 5:41 PM, Pengjie Zhang wrote:
>>>>> The legacy CPPC feedback-counter path reads the delivered and 
>>>>> reference
>>>>> performance counters separately.
>>>>>
>>>>> On arm64 systems using AMU-backed CPPC FFH counters, each FFH read is
>>>>> served through a cross-CPU counter read helper. Reading the counters
>>>>> separately therefore widens the sampling window between them and can
>>>>> skew the delivered/reference ratio used by cpuinfo_cur_freq. Under 
>>>>> heavy
>>>>> load, the skew is observable as transient values that may exceed the
>>>>> platform maximum, as discussed in [1] and [2].
>>>>>
>>>>> This series adds a small generic hook for architectures that can 
>>>>> obtain
>>>>> both FFH feedback counters in one operation, while preserving the
>>>>> existing per-register read path as the fallback.
>>>>>
>>>>> Patch 1 adds the generic CPPC hook and uses it from 
>>>>> cppc_get_perf_ctrs().
>>>>> Patch 2 implements the hook on arm64 by sampling both AMU counters 
>>>>> in a
>>>>> single operation on the target CPU.
>>>>>
>>>>> [1] 
>>>>> https://lore.kernel.org/all/20231025093847.3740104-4-zengheng4@huawei.com/
>>>>> [2] 
>>>>> https://lore.kernel.org/all/20231212072617.14756-1-lihuisong@huawei.com/ 
>>>>>
>>>>>
>>>>> Signed-off-by: Pengjie Zhang <zhangpengjie2@huawei.com>
>>>>>
>>>>> Pengjie Zhang (2):
>>>>>    ACPI: CPPC: add paired FFH feedback-counter read hook
>>>>>    arm64: topology: read CPPC FFH feedback counters in one operation
>>>>>
>>>>>   arch/arm64/kernel/topology.c | 75 
>>>>> ++++++++++++++++++++++++++++++++----
>>>>>   drivers/acpi/cppc_acpi.c     | 58 +++++++++++++++++++++++++---
>>>>>   include/acpi/cppc_acpi.h     |  7 ++++
>>>>>   3 files changed, 127 insertions(+), 13 deletions(-)
>>>>>
>