Re: [PATCH v2 0/2] CPPC: reduce FFH feedback-counter sampling skew on arm64

[Pengjie Zhang]

Hi Rafael,

Just a gentle ping on this patch. It has been a week and Beata has taken
a look at it. Could you please let me know if you have any comments or
concerns regarding this change?

Thanks,
    Pengjie

On 7/10/2026 9:42 PM, Beata Michalska wrote:
> Looks good to me, though I guess you might need an ack from Rafael (?)
>
> Thank you for the patches.
>
> ---
> BR
> Beata
> On Wed, Jul 08, 2026 at 04:28:16PM +0800, 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.
> >
> > For detailed test results and data demonstrating the observable skew and
> > the improvements brought by this series, please refer to [3] and [4].
> >
> > [1] https://lore.kernel.org/all/20231025093847.3740104-4-zengheng4@xxxxxxxxxx/
> > [2] https://lore.kernel.org/all/20231212072617.14756-1-lihuisong@xxxxxxxxxx/
> > [3] https://lore.kernel.org/all/443104e2-ba6e-454e-8469-909f35817a99@xxxxxxxxxx/
> > [4] https://lore.kernel.org/all/317d33d5-8279-4aa8-84b7-6ae1976636ac@xxxxxxxxxx/
> >
> > Tested-by: Sumit Gupta <sumitg@xxxxxxxxxx>
> > Reviewed-by: Sumit Gupta <sumitg@xxxxxxxxxx>
> > Tested-by: Vanshidhar Konda <vanshikonda@xxxxxxxxxxxxxxxxxxxxxx>
> > Reviewed-by: Vanshidhar Konda <vanshikonda@xxxxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Pengjie Zhang <zhangpengjie2@xxxxxxxxxx>
> > ---
> > Changes in v2:
> > - Simplified the CPPC generic layer fallback logic to prevent pointless single-read retries.
> > - Added upfront register validation in the arm64 hook to avoid unnecessary IPI overhead.
> > - Explicitly flipped the -EOPNOTSUPP error to -ENODEV in the arm64 hook when AMU is unsupported, cleanly bypassing redundant CPPC generic fallbacks.
> > - Addressed other kernel-doc and naming feedbacks from Beata.
> > - Added Reviewed-by and Tested-by tags from Vanshidhar and Sumit
> > - Link to v1: https://lore.kernel.org/all/20260410094145.4132082-1-zhangpengjie2@xxxxxxxxxx/
> >
> > 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 | 92 ++++++++++++++++++++++++++++++++----
> >   drivers/acpi/cppc_acpi.c     | 50 ++++++++++++++++++--
> >   include/acpi/cppc_acpi.h     |  7 +++
> >   3 files changed, 136 insertions(+), 13 deletions(-)
> >
> > --
> > 2.33.0