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Subject[PATCH v2 0/2] Selftest for cpuidle latency measurement
Date
v1: https://lkml.org/lkml/2020/7/7/1036
Changelog v1 --> v2
1. Based on Shuah Khan's comment, changed exit code to ksft_skip to
indicate the test is being skipped
2. Change the busy workload for baseline measurement from
"yes > /dev/null" to "cat /dev/random to /dev/null", based on
observed CPU utilization for "yes" consuming ~60% CPU while the
latter consumes 100% of CPUs, giving more accurate baseline numbers
---

The patch series introduces a mechanism to measure wakeup latency for
IPI and timer based interrupts
The motivation behind this series is to find significant deviations
behind advertised latency and resisdency values

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,
1. 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
2. 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
To include the module, check option and include as module
kernel hacking -> Cpuidle latency selftests

The selftest inserts the module, disables all the idle states and
enables them one by one testing the following:
1. Keeping source CPU constant, iterates through all the CPUS measuring
IPI latency for baseline (CPU is busy with
"cat /dev/random > /dev/null" workload) and the when the CPU is
allowed to be at rest
2. Iterating through all the CPUs, sending expected timer durations to
be equivalent to the residency of the the deepest idle state
enabled and extracting the difference in time between the time of
wakeup and the expected timer duration

Usage
-----
Can be used in conjuction to the rest of the selftests.
Default Output location in: tools/testing/cpuidle/cpuidle.log

To run this test specifically:
$ make -C tools/testing/selftests TARGETS="cpuidle" run_tests

There are a few optinal arguments too that the script can take
[-h <help>]
[-m <location of the module>]
[-o <location of the output>]

Sample output snippet
---------------------
--IPI Latency Test---
--Baseline IPI Latency measurement: CPU Busy--
SRC_CPU DEST_CPU IPI_Latency(ns)
...
0 8 1996
0 9 2125
0 10 1264
0 11 1788
0 12 2045
Baseline Average IPI latency(ns): 1843
---Enabling state: 5---
SRC_CPU DEST_CPU IPI_Latency(ns)
0 8 621719
0 9 624752
0 10 622218
0 11 623968
0 12 621303
Expected IPI latency(ns): 100000
Observed Average IPI latency(ns): 622792

--Timeout Latency Test--
--Baseline Timeout Latency measurement: CPU Busy--
Wakeup_src Baseline_delay(ns)
...
8 2249
9 2226
10 2211
11 2183
12 2263
Baseline Average timeout diff(ns): 2226
---Enabling state: 5---
8 10749
9 10911
10 10912
11 12100
12 73276
Expected timeout(ns): 10000200
Observed Average timeout diff(ns): 23589

Pratik Rajesh Sampat (2):
cpuidle: Trace 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 | 150 ++++++++++++
lib/Kconfig.debug | 10 +
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/cpuidle/Makefile | 6 +
tools/testing/selftests/cpuidle/cpuidle.sh | 257 +++++++++++++++++++++
tools/testing/selftests/cpuidle/settings | 1 +
7 files changed, 426 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

--
2.25.4

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