Re: [PATCH] cpufreq: cppc: Reduce cppc delivered perf sampling jitter

[Breno Leitao <leitao@debian.org>]

On Tue, Jun 02, 2026 at 04:20:52PM -0500, Jeremy Linton wrote:
> CPPC uses a pair of registers cycling at different frequencies to
> determine an accumulated performance level. For userspace reporting we
> want to convert this to an instantaneous CPU frequency, but over short
> time periods small errors caused by CPPC counter reads can cause
> fairly significant reported frequency variations even when the core
> CPU clock isn't changing.
> 
> Reduce this by keeping a start sample fixed and retrying the end
> sample until the counter deltas are large enough to reduce short
> window error, or until adjacent delivered performance estimates are
> within the CPU's observed CPPC read noise floor.
> 
> To begin, resample the initial pair a small fixed number of times
> looking for matching delivered performance deltas.  This reduces the
> chance that a disturbed start sample anchors the rest of the
> calculation.
> 
> Then look for an end sample while updating the noise floor from the
> best error seen between samples.  The floor remains zero on systems
> with stable feedback reads, but lets noisy systems stop early once
> another retry is unlikely to improve the result.  The retry loop is
> capped at 200 iterations, giving an ~20 usec explicit delay budget
> derived from ndelay(100).
> 
> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>

Tested-by: Breno Leitao <leitao@debian.org>

I've tested this on my grace host, checking "cat
/sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq" thousand times,
and the result is quite good, reducing the Std Dev by 5x, and getting
a better value.

# cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq
 3339000

Baseline:
	Average: 3505050.00
	P50 (Median): 3388500.00
	Std Dev: 462607.66
	Min: 2709000.00
	Max: 4464000.00


Wth this patch:
	Average: 2393550.00
	P50 (Median): 2394000.00
	Std Dev: 68629.06
	Min: 2241000.00
	Max: 2529000.00

Thanks,
--breno