RE: [patch 00/10] x86/cpu: Consolidate APERF/MPERF code

Doug Smythies
2022-04-19T17:32:43Z

Hi Thomas,

On 2022.04.15 12:20 Thomas Gleixner wrote:

> APERF/MPERF is utilized in two ways:
>
> 1) Ad hoc readout of CPU frequency which requires IPIs
>
> 2) Frequency scale calculation for frequency invariant scheduling which
> reads APERF/MPERF on every tick.
>
> These are completely independent code parts. Eric observed long latencies
> when reading /proc/cpuinfo which reads out CPU frequency via #1 and
> proposed to replace the per CPU single IPI with a broadcast IPI.
>
> While this makes the latency smaller, it is not necessary at all because #2
> samples APERF/MPERF periodically, except on idle or isolated NOHZ full CPUs
> which are excluded from IPI already.
>
> It could be argued that not all APERF/MPERF capable systems have the
> required BIOS information to enable frequency invariance support, but in
> practice most of them do. So the APERF/MPERF sampling can be made
> unconditional and just the frequency scale calculation for the scheduler
> excluded.
>
> The following series consolidates that.

I have used this patch set with the acpi-cpufreq, intel_cpufreq (passive),
and intel_pstate (active) CPU frequency scaling drivers and various
governors. Additionally, with HWP both enabled and disabled.

For intel_pstate (active), both HWP enabled or disabled, the behaviour
of scaling_cur_freq is inconsistent with prior to this patch set and other
scaling driver governor combinations.

Note there is no issue with " grep MHz /proc/cpuinfo" for any
combination.

Examples:

No-HWP:

active/powersave:
doug@s19:~/freq-scalers/trace$ grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
/sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq:2300418
/sys/devices/system/cpu/cpu10/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu11/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu1/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu2/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu3/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu4/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu5/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu6/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu7/cpufreq/scaling_cur_freq:2300006
/sys/devices/system/cpu/cpu8/cpufreq/scaling_cur_freq:2300005
/sys/devices/system/cpu/cpu9/cpufreq/scaling_cur_freq:0

active/performance:
doug@s19:~$ grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
/sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu10/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu11/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu1/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu2/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu3/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu4/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu5/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu6/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu7/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu8/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu9/cpufreq/scaling_cur_freq:0

HWP:

active/powersave:
doug@s19:~$ grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
/sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu10/cpufreq/scaling_cur_freq:799993
/sys/devices/system/cpu/cpu11/cpufreq/scaling_cur_freq:800069
/sys/devices/system/cpu/cpu1/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu2/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu3/cpufreq/scaling_cur_freq:800131
/sys/devices/system/cpu/cpu4/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu5/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu6/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu7/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu8/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu9/cpufreq/scaling_cur_freq:799844

active/performance:

doug@s19:~$ grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
/sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu10/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu11/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu1/cpufreq/scaling_cur_freq:4800186
/sys/devices/system/cpu/cpu2/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu3/cpufreq/scaling_cur_freq:4800016
/sys/devices/system/cpu/cpu4/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu5/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu6/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu7/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu8/cpufreq/scaling_cur_freq:0
/sys/devices/system/cpu/cpu9/cpufreq/scaling_cur_freq:0

Other configurations:
intel_cpufreq /schedutil (no HWP), for example:

doug@s19:~$ grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
/sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq:800000
/sys/devices/system/cpu/cpu10/cpufreq/scaling_cur_freq:1067573
/sys/devices/system/cpu/cpu11/cpufreq/scaling_cur_freq:800000
/sys/devices/system/cpu/cpu1/cpufreq/scaling_cur_freq:800000
/sys/devices/system/cpu/cpu2/cpufreq/scaling_cur_freq:800000
/sys/devices/system/cpu/cpu3/cpufreq/scaling_cur_freq:800011
/sys/devices/system/cpu/cpu4/cpufreq/scaling_cur_freq:800000
/sys/devices/system/cpu/cpu5/cpufreq/scaling_cur_freq:800109
/sys/devices/system/cpu/cpu6/cpufreq/scaling_cur_freq:800000
/sys/devices/system/cpu/cpu7/cpufreq/scaling_cur_freq:800000
/sys/devices/system/cpu/cpu8/cpufreq/scaling_cur_freq:800000
/sys/devices/system/cpu/cpu9/cpufreq/scaling_cur_freq:800000

Processor: Intel(R) Core(TM) i5-10600K CPU @ 4.10GHz

> Thanks,
>
> tglx
> ---
> arch/x86/include/asm/cpu.h | 2
> arch/x86/include/asm/topology.h | 17 -
> arch/x86/kernel/acpi/cppc.c | 28 --
> arch/x86/kernel/cpu/aperfmperf.c | 474 +++++++++++++++++++++++++++++++--------
> arch/x86/kernel/cpu/proc.c | 2
> arch/x86/kernel/smpboot.c | 358 -----------------------------
> fs/proc/cpuinfo.c | 6
> include/linux/cpufreq.h | 1
> 8 files changed, 405 insertions(+), 483 deletions(-)