CPU-Idle latency selftest framework

Changelog RFC v4 --> PATCH v5:
1. Added a CPU online check prior to parsing the CPU topology to avoid parsing topologies for CPUs unavailable for the latency test
2. Added comment describing the selftest in cpuidle.sh

As I have made changes to cpuidle.sh's working, hence dropping "Reviewed-by" from Doug Smythies for the second patch, while retaining it for the first patch.

RFC v4: https://lkml.org/lkml/2021/4/12/99
---
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 R. 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 | 414 +++++++++++++++++++++
 tools/testing/selftests/cpuidle/settings   |   2 +
 7 files changed, 591 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

2.17.1

Introduce a mechanism to fire directed IPIs from a specified source CPU to a specified target CPU and measure the difference in time incurred on wakeup.

Also, introduce a mechanism to queue a HR timer on a specified CPU and subsequently measure the time taken to wakeup the CPU.

Finally define a simple debugfs interface to control the knobs to fire the IPI and Timer events on specified CPU and view their incurred idle wakeup latencies.

---
 drivers/cpuidle/Makefile               |   1 +
 drivers/cpuidle/test-cpuidle_latency.c | 157 +++++++++++++++++++++++++
 lib/Kconfig.debug                      |  10 ++
 3 files changed, 168 insertions(+)
 create mode 100644 drivers/cpuidle/test-cpuidle_latency.c

index 26bbc5e74123..3b4ee06a9164 100644
@@ -8,6 +8,7 @@ obj-$(CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED) += coupled.o
 obj-$(CONFIG_DT_IDLE_STATES)		  += dt_idle_states.o
 obj-$(CONFIG_ARCH_HAS_CPU_RELAX)	  += poll_state.o
 obj-$(CONFIG_HALTPOLL_CPUIDLE)		  += cpuidle-haltpoll.o
+obj-$(CONFIG_IDLE_LATENCY_SELFTEST)	  += test-cpuidle_latency.o
 
 ##################################################################################
 # ARM SoC drivers

index 417c3d3e521b..70264687e0c2 100644
@@ -1513,6 +1513,16 @@ config DEBUG_KOBJECT
 	  If you say Y here, some extra kobject debugging messages will be sent
 	  to the syslog.
 
+config IDLE_LATENCY_SELFTEST
+	tristate "Cpuidle latency selftests"
+	depends on CPU_IDLE
+	help
+	  This option provides a kernel module that runs tests using the IPI and
+	  timers to measure latency.
+
+	  Say M if you want these self tests to build as a module.
+	  Say N if you are unsure.
+
 config DEBUG_KOBJECT_RELEASE
 	bool "kobject release debugging"
 	depends on DEBUG_OBJECTS_TIMERS

2.17.1

The cpuidle latency selftest provides support to systematically extract, analyse and present IPI and timer based wakeup latencies for each CPU and each idle state available on the system.

The selftest leverages test-cpuidle_latency module's debugfs interface to interact and extract latency information from the kernel.

The selftest inserts the module if already not inserted, disables all the idle states and enables them one by one testing the following:
1. Keeping source CPU constant, iterate through all the CPUS measuring IPI latency for baseline (CPU is busy with cat /dev/random > /dev/null workload) and then 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 deepest idle state enabled and extracting the difference in time between the time of wakeup and the expected timer duration

The timer based test produces run to run variance on some intel based systems that sport a mechansim "C-state pre-wake" which can pre-wake a CPU from an idle state when timers are armed. For systems and architectures that don't have this mechansim can leverage timer tests with the -i option.

To run this test specifically:

$ sudo make -C tools/testing/selftests TARGETS="cpuidle" run_tests

There are a few optional arguments too that the script can take
[-h <help>]
[-i <run timer tests>]
[-m <location of the module>]
[-o <location of the output>]
[-v <verbose> (run on all cpus)]
Default Output location in: tools/testing/selftest/cpuidle/cpuidle.log

To run the test without re-compiling:

$ cd tools/testing/selftest/cpuidle/
$ sudo ./cpuidle.sh

---
 tools/testing/selftests/Makefile           |   1 +
 tools/testing/selftests/cpuidle/Makefile   |   6 +
 tools/testing/selftests/cpuidle/cpuidle.sh | 414 +++++++++++++++++++++
 tools/testing/selftests/cpuidle/settings   |   2 +
 4 files changed, 423 insertions(+)
 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

index 6c575cf34a71..561eb67a4839 100644
@@ -7,6 +7,7 @@ TARGETS += cgroup
 TARGETS += clone3
 TARGETS += core
 TARGETS += cpufreq
+TARGETS += cpuidle
 TARGETS += cpu-hotplug
 TARGETS += drivers/dma-buf
 TARGETS += efivarfs

new file mode 100644
index 000000000000..72fd5d2e974d
@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: GPL-2.0
+all:
+
+TEST_PROGS := cpuidle.sh
+
+include ../lib.mk

new file mode 100644
index 000000000000..a26c38a70d77
@@ -0,0 +1,2 @@
+timeout=0
+

2.17.1

Hi @Rafael and @Shuah,

Gentle ping.

Is there any feedback on this patch-set?

Quick summary and history:
1. The patchset introduces a kernel module and a bash selftest driver to estimate wakeup latency caused by entering idle states
2. The patchset has seemed to provide useful feedback on latency of idle states on the IBM POWER architecture
3. It also seems to also be providing desirable results on Intel machines with the IPI mechanism (Timer tests are optional here due to some Intel processors having a pre-wakeup feature and may not tend to actual idle latency) as reviewed by Doug Smythies.
   Intel numbers for reference: https://lkml.org/lkml/2021/4/13/785

--
Thanks
Pratik