Re: [RESEND][PATCH 0/3] ACPI: CPPC: Reduce .target() callback overhead

[Rafael J. Wysocki (Intel)]

On Tue, Aug 4, 2026 at 12:28 AM Christian Loehle
<christian.loehle@xxxxxxx> wrote:
>
> On 8/3/26 22:05, Christian Loehle wrote:
> > cppc-cpufreq reaches cppc_set_perf() from every target callback.  For
> > direct SystemMemory controls, that path currently does several steps
> > which are unnecessary once the immutable _CPC layout is known:
> >
> >   - a full-width write first reads the access unit and merges the value.
> >   - every write takes the descriptor's RMW lock, even when its access unit
> >     is not shared with another _CPC entry.
> >   - cppc_set_perf() evaluates the same three PCC predicates at each phase
> >     of the transaction.
> >
> > Remove those costs while retaining the existing conservative paths for
> > partial fields, overlapping or malformed access units, and PCC controls.
> >
> > The series was tested on Arm Power-Orion O6 and AmpereOne systems using
> > cppc-cpufreq and schedutil.  An rt-app task pinned to one CPU ran for
> > 500 us every 2 ms with uclamp.min=512, for 5000 periods per run.  schedutil
> > rate_limit_us was 1000.  cppc_cpufreq_fast_switch() latency was measured
> > for 100 ten-second runs.  Each sample is the mean callback latency within
> > one run, and the table reports the median and sample standard deviation
> > of those samples.
> >
> > Orion O6               median       stdev       callbacks
> > baseline               5703 ns      307 ns       188017
> > complete series        5023 ns      240 ns       188209
> > ==> 680 ns (11.9%) reduction
> >
> > AmpereOne              median       stdev
> > baseline               2090 ns      157 ns
> > complete series      1907.5 ns      140 ns
> > ==> 182.5 ns (8.7%) reduction
> >
> > The cumulative intermediate results on the Orion O6 attribute roughly half
> > of the gain to each of the first two patches: avoiding the read reduced
> > the median by 284 ns (5.0%), and avoiding the lock reduced it by another
> > 336 ns (6.2%).
> > Together they account for 620 ns of the 680 ns total reduction.
> >
> > With the same arm64 configuration and GCC 11.4, caching the PCC predicate
> > also reduces cppc_set_perf() from 1124 to 884 bytes.  The generated
> > function has 60 fewer instructions, 17 fewer loads and 20 fewer branches.
> >
> > This series is based on the CPPC fixes posted at (already queued):
> > https://lore.kernel.org/lkml/20260722093825.1030594-1-christian.loehle@xxxxxxx/
> >
> > and the separately posted fix still under review:
> > https://lore.kernel.org/lkml/20260724104042.1481804-1-christian.loehle@xxxxxxx/
> >
> > PS:
> > There's a final optimization that I actually wanted to make but decided to
> > split it out for now as it somewhat replicated Sumit's series:
> > Skipping redundant perf ctrl writes in cpufreq-cppc if registers are non-PCC
> > and !shared (because the values are unchanged, the
> > !autonomous-common-case), but that requires the driver to have a more
> > complex caching- and atomic-updating machinery in place, similar to
> > hotplug. As opposed to this series the optimization would be for the
> > microcontroller handling the CPPC requests, which may be shared across
> > many CPUs and therefore redundant requests can increase the dvfs
> > transition latency.
> > That patch will follow once Sumit's is queued:
> > https://lore.kernel.org/lkml/20260716153820.2007095-1-sumitg@xxxxxxxxxx/
> >
> > [RESEND] for the new base-commit specifier below
> >
> > Christian Loehle (3):
> >   ACPI: CPPC: Avoid unnecessary reads for full-width writes
> >   ACPI: CPPC: Avoid locking standalone full-width registers
> >   ACPI: CPPC: Evaluate performance-control PCC use once
> >
> >  drivers/acpi/cppc_acpi.c | 135 +++++++++++++++++++++++++++++++--------
> >  include/acpi/cppc_acpi.h |   5 +-
> >  2 files changed, 113 insertions(+), 27 deletions(-)
> >
> >
> > base-commit: 075b74841bd0065a3bda3440873c747938e69b68
>
> Nice, so that worked:
> https://sashiko.dev/#/patchset/20260803210527.1285229-1-christian.loehle@xxxxxxx
> Seems no findings on $SUBJECT

Yes, so applied as 7.3 material.

> but the rest of the comments it has look legit to me?

Yes, but please address those on top of the $subject series, thanks!

> I'll go take another look tomorrow:
>
> 1. Using per-CPU cpc_desc->rmw_lock for SYSTEM_MEMORY CPPC control register that
> may be shared.
> 2. MASK_VAL_WRITE() truncation on 32bit architectures.
> 3. acpi_cppc_processor_exit() calls kfree(cpc_ptr) unconditionally (UAF with sysfs?)
> 4. cppc_set_reg_val_in_pcc() calls cpc_write() before down_write(&pcc_ss_data->pcc_lock)
> 5 & 6 are sanitization of values from FW.

Sounds good to me.