[RFC,v4,0/2] CPU-Idle latency selftest framework

Changelog v3-->v4
Based on review comments by Doug Smythies,
1. Parsing the thread_siblings_list for CPU topology information to
   correctly identify the cores the test should run on in
   default(quick) mode.
2. The source CPU to source CPU interaction in the IPI test will always
   result in a lower latency and cause a bias in the average, hence
   avoid adding the latency to be averaged for same cpu IPIs. The
   latency will still be displayed in the detailed logs.

RFC v3: https://lkml.org/lkml/2021/4/4/31
---
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 | 402 +++++++++++++++++++++
 tools/testing/selftests/cpuidle/settings   |   2 +
 7 files changed, 579 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

Hi Pratik,

On Mon, Apr 12, 2021 at 12:43 AM Pratik Rajesh Sampat
<psampat@linux.ibm.com> wrote:
>

> 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.

>

> Signed-off-by: Pratik Rajesh Sampat <psampat@linux.ibm.com>


Reviewed-by: Doug Smythies <dsmythies@telus.net>