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