CPU-Idle latency selftest framework
Subsystems (3)
v2 cover CPU-Idle latency selftest framework
Changelog
RFC v1-->v2
The timer based test produces run to run variance on some intel based systems that sport a mechansim of "C-state pre-wake" which can pre-wake a CPU from an idle state when timers are armed. Hence invoking the timer tests is now parameterized for systems and architectures that don't support pre-wakeup logic and need granular timer measurements along with IPI results.
This RFC does not yet support treating of CPU 0s idle states differently especially as reported on Intel systems. More understanding is needed on systems to determine if only CPU 0 is treated differently of if they are more CPUs that cannot have its idle state properties changed.
RFC v1: https://lkml.org/lkml/2021/3/15/492
---
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 | 323 +++++++++++++++++++++ tools/testing/selftests/cpuidle/settings | 2 + 7 files changed, 500 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
Signature · 1 line
2.17.1
RFC v2 1/2 cpuidle: Extract IPI based and timer based wakeup latency from idle states
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
drivers/cpuidle/Makefile +1 -0
@@ -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
drivers/cpuidle/test-cpuidle_latency.c new file +157 -0
157 lines not on this page · raw message
lib/Kconfig.debug +10 -0
@@ -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
Signature · 1 line
2.17.1
RFC v2 2/2 selftest/cpuidle: Add support for cpuidle latency measurement
In reply to Pratik Rajesh SampatThe 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:
$ make -C tools/testing/selftests TARGETS="cpuidle" run_tests
There are a few optinal 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
--- tools/testing/selftests/Makefile | 1 + tools/testing/selftests/cpuidle/Makefile | 6 + tools/testing/selftests/cpuidle/cpuidle.sh | 323 +++++++++++++++++++++ tools/testing/selftests/cpuidle/settings | 2 + 4 files changed, 332 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
tools/testing/selftests/Makefile +1 -0
@@ -7,6 +7,7 @@ TARGETS += cgroup TARGETS += clone3 TARGETS += core TARGETS += cpufreq +TARGETS += cpuidle TARGETS += cpu-hotplug TARGETS += drivers/dma-buf TARGETS += efivarfs
tools/testing/selftests/cpuidle/Makefile new file +6 -0
@@ -0,0 +1,6 @@ +# SPDX-License-Identifier: GPL-2.0 +all: + +TEST_PROGS := cpuidle.sh + +include ../lib.mk
tools/testing/selftests/cpuidle/cpuidle.sh new file +323 -0
323 lines not on this page · raw message
tools/testing/selftests/cpuidle/settings new file +2 -0
@@ -0,0 +1,2 @@ +timeout=0 +
Signature · 1 line
2.17.1
Re: [RFC v2 2/2] selftest/cpuidle: Add support for cpuidle latency measurement
Hi Pratik,
Quoted 1 line · Pratik Rajesh Sampat
...
Quoted 2 lines
To run this test specifically:
$ make -C tools/testing/selftests TARGETS="cpuidle" run_tests
I have not become any smarter than I was with version 1, and still assumed that the "$" meant regular user. Please put it as "#" or separate the two steps, compile and run.
Quoted 2 lines
There are a few optinal arguments too that the script can take
optional
Suggest to also specifically mention how to run without re-compile,
# ./cpuidle.sh -v
Note also that the test still leaves all idle states disabled when done.
Quoted 6 lines
[-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
...
tools/testing/selftests/cpuidle/cpuidle.sh line 136
Quoted 5 lines
tools/testing/selftests/cpuidle/cpuidle.sh new file +5
-0+cpu_is_online() +{ + cpu=$1 + if [ ! -f "/sys/devices/system/cpu/cpu$cpu/online" ]; then + echo 0
incorrect. should be:
Quoted 1 line
+ echo 1
... Doug
Re: [RFC v2 2/2] selftest/cpuidle: Add support for cpuidle latency measurement
Hello Doug,
Thanks for your review.
Quoted 11 lines · Doug Smythies
Hi Pratik,
On Thu, Apr 1, 2021 at 4:45 AM Pratik Rajesh Sampat <[email protected]> wrote:
...Quoted 2 lines
To run this test specifically:
$ make -C tools/testing/selftests TARGETS="cpuidle" run_testsI have not become any smarter than I was with version 1, and still assumed that the "$" meant regular user. Please put it as "#" or separate the two steps, compile and run.
Apologies, I missed incorporating the root user discussion we had. I'll add a sudo past the "$" symbol.
Quoted 6 lines
Quoted 1 line
There are a few optinal arguments too that the script can take
optional
Suggest to also specifically mention how to run without re-compile,
# ./cpuidle.sh -v
Sure thing, I'll add a comment specifying this.
Quoted 1 line
Note also that the test still leaves all idle states disabled when done.
Yes, I missed out enabling all the idle states after the tests are done. I'll spin a new version where I enable idle states at the end of the experiment so that the system stays coherent.
Quoted 6 lines
Quoted 6 lines
[-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
Quoted 10 lines
...
tools/testing/selftests/cpuidle/cpuidle.sh line 136
Quoted 5 lines
tools/testing/selftests/cpuidle/cpuidle.sh new file +5
-0+cpu_is_online() +{ + cpu=$1 + if [ ! -f "/sys/devices/system/cpu/cpu$cpu/online" ]; then + echo 0incorrect. should be:
Quoted 1 line
+ echo 1
Right! Thanks for catching this.
Quoted 1 line
... Doug
Thank you,
Pratik