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) - State0: 3265 # Observed Average IPI latency(ns) - State1: 3507 # Observed Average IPI latency(ns) - State2: 3739 # Observed Average IPI latency(ns) - State3: 3807 # Observed Average IPI latency(ns) - State4: 17070 # Observed Average IPI latency(ns) - State5: 1038174 # Observed Average IPI latency(ns) - State6: 1068784 # # --Timeout Latency Test-- # Baseline Average timeout diff(ns): 1420 # Observed Average timeout diff(ns) - State0: 1640 # Observed Average timeout diff(ns) - State1: 1764 # Observed Average timeout diff(ns) - State2: 1715 # Observed Average timeout diff(ns) - State3: 1845 # Observed Average timeout diff(ns) - State4: 16581 # Observed Average timeout diff(ns) - State5: 939977 # Observed Average timeout diff(ns) - State6: 1073024
Things to keep in mind:
1. This kernel module + bash driver does not guarantee idleness on a core when the IPI and the Timer is armed. It only invokes sleep and hopes that the core is idle once the IPI/Timer is invoked onto it. Hence this program must be run on a completely idle system for best results
2. Even on a completely idle system, there maybe book-keeping tasks or jitter tasks that can run on the core we want idle. This can create outliers in the latency measurement. Thankfully, these outliers should be large enough to easily weed them out.
3. A userspace only selftest variant was also sent out as RFC based on suggestions over the previous patchset to simply the kernel complexeity. However, a userspace only approach had more noise in the latency measurement due to userspace-kernel interactions which led to run to run variance and a lesser accurate test. Another downside of the nature of a userspace program is that it takes orders of magnitude longer to complete a full system test compared to the kernel framework. RFC patch: https://lkml.org/lkml/2020/9/2/356
4. For Intel Systems, the Timer based latencies don't exactly give out the measure of idle latencies. This is because of a hardware optimization mechanism that pre-arms a CPU when a timer is set to wakeup. That doesn't make this metric useless for Intel systems, it just means that is measuring IPI/Timer responding latency rather than idle wakeup latencies. (Source: https://lkml.org/lkml/2020/9/2/610) For solution to this problem, a hardware based latency analyzer is devised by Artem Bityutskiy from Intel. https://youtu.be/Opk92aQyvt0?t=8266 https://intel.github.io/wult/
Pratik Rajesh Sampat (2): cpuidle: Extract IPI based and timer based wakeup latency from idle states selftest/cpuidle: Add support for cpuidle latency measurement
drivers/cpuidle/Makefile | 1 + drivers/cpuidle/test-cpuidle_latency.c | 157 ++++++++++ lib/Kconfig.debug | 10 + tools/testing/selftests/Makefile | 1 + tools/testing/selftests/cpuidle/Makefile | 6 + tools/testing/selftests/cpuidle/cpuidle.sh | 316 +++++++++++++++++++++ tools/testing/selftests/cpuidle/settings | 2 + 7 files changed, 493 insertions(+) create mode 100644 drivers/cpuidle/test-cpuidle_latency.c create mode 100644 tools/testing/selftests/cpuidle/Makefile create mode 100755 tools/testing/selftests/cpuidle/cpuidle.sh create mode 100644 tools/testing/selftests/cpuidle/settings