[RFC 0/2] CPU-Idle latency selftest framework

[Pratik R. Sampat]

A kernel module + userspace driver to estimate the wakeup latency
caused by going into stop states. The motivation behind this program is
to find significant deviations behind advertised latency and residency
values.

The patchset measures latencies for two kinds of events. IPIs and Timers
As this is a software-only mechanism, there will additional latencies of
the kernel-firmware-hardware interactions. To account for that, the
program also measures a baseline latency on a 100 percent loaded CPU
and the latencies achieved must be in view relative to that.

To achieve this, we introduce a kernel module and expose its control
knobs through the debugfs interface that the selftests can engage with.

The kernel module provides the following interfaces within
/sys/kernel/debug/latency_test/ for,

IPI test:
    ipi_cpu_dest = Destination CPU for the IPI
    ipi_cpu_src = Origin of the IPI
    ipi_latency_ns = Measured latency time in ns
Timeout test:
    timeout_cpu_src = CPU on which the timer to be queued
    timeout_expected_ns = Timer duration
    timeout_diff_ns = Difference of actual duration vs expected timer


Sample output on a POWER9 system is as follows:
# --IPI Latency Test---
# Baseline Average IPI latency(ns): 3114
# Observed Average IPI latency(ns) - Snooze: 3265
# Observed Average IPI latency(ns) - Stop0_lite: 3507
# Observed Average IPI latency(ns) - Stop0: 3739
# Observed Average IPI latency(ns) - Stop2: 3807
# Observed Average IPI latency(ns) - Stop4: 17070
# Observed Average IPI latency(ns) - Stop5: 1038174
# 
# --Timeout Latency Test--
# Baseline Average timeout diff(ns): 1420
# Observed Average timeout diff(ns) - Snooze: 1640
# Observed Average timeout diff(ns) - Stop0_lite: 1764
# Observed Average timeout diff(ns) - Stop0: 1715
# Observed Average timeout diff(ns) - Stop2: 1845
# Observed Average timeout diff(ns) - Stop4: 16581
# Observed Average timeout diff(ns) - Stop5: 939977

Pratik R. Sampat (2):
  powerpc/cpuidle: Extract IPI based and timer based wakeup latency from
    idle states
  powerpc/selftest: Add support for cpuidle latency measurement

 arch/powerpc/kernel/Makefile                  |   1 +
 arch/powerpc/kernel/test-cpuidle_latency.c    | 157 +++++++
 lib/Kconfig.debug                             |  10 +
 tools/testing/selftests/powerpc/Makefile      |   1 +
 .../powerpc/cpuidle_latency/.gitignore        |   2 +
 .../powerpc/cpuidle_latency/Makefile          |   6 +
 .../cpuidle_latency/cpuidle_latency.sh        | 419 ++++++++++++++++++
 .../powerpc/cpuidle_latency/settings          |   1 +
 8 files changed, 597 insertions(+)
 create mode 100644 arch/powerpc/kernel/test-cpuidle_latency.c
 create mode 100644 tools/testing/selftests/powerpc/cpuidle_latency/.gitignore
 create mode 100644 tools/testing/selftests/powerpc/cpuidle_latency/Makefile
 create mode 100755 tools/testing/selftests/powerpc/cpuidle_latency/cpuidle_latency.sh
 create mode 100644 tools/testing/selftests/powerpc/cpuidle_latency/settings

-- 
2.17.1