linux-kernel - [RFC v5 0/1] Selftest for cpuidle latency measurement

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Date:   Sun,  4 Oct 2020 17:41:13 +0530
From:   Pratik Rajesh Sampat <psampat@...ux.ibm.com>
To:     rjw@...ysocki.net, daniel.lezcano@...aro.org,
        srivatsa@...il.mit.edu, shuah@...nel.org, ego@...ux.vnet.ibm.com,
        svaidy@...ux.ibm.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        pratik.r.sampat@...il.com, psampat@...ux.ibm.com
Subject: [RFC v5 0/1] Selftest for cpuidle latency measurement

v4: https://lkml.org/lkml/2020/9/2/356
v4-->v5
Based on comments from Artem Bityutskiy, evaluation of timer based
wakeup latencies may not be a fruitful measurement especially on the x86
platform which has the capability to pre-arm a CPU when a timer is set.

Hence, including only the IPI based tests for latency measurement to
acheive expected behaviour across platforms.

kernel module + bash selftest approach which presents lower deviations
and higher accuracy: https://lkml.org/lkml/2020/7/21/567
---

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

To achieve this in the userspace, IPI latencies are calculated by
sending information through pipes and inducing a wakeup.

To account for delays from kernel-userspace interactions baseline
observations are taken on a 100% busy CPU and subsequent obervations
must be considered relative to that.

One downside of the userspace approach in contrast to the kernel
implementation is that the run to run variance can turn out to be high
in the order of ms; which is the scope of the experiments at times.

Another downside of the userspace approach is that it takes much longer
to run and hence a command-line option quick and full are added to make
sure quick 1 CPU tests can be carried out when needed and otherwise it
can carry out a full system comprehensive test.

Usage
---
./cpuidle --mode <full / quick / num_cpus> --output <output location> 
full: runs on all CPUS
quick: run on a random CPU
num_cpus: Limit the number of CPUS to run on

Sample output snippet
---------------------
--IPI Latency Test---
SRC_CPU   DEST_CPU IPI_Latency(ns)
...
  0          5       256178
  0          6       478161
  0          7       285445
  0          8       273553
Expected IPI latency(ns): 100000
Observed Average IPI latency(ns): 248334

Pratik Rajesh Sampat (1):
  selftests/cpuidle: Add support for cpuidle latency measurement

 tools/testing/selftests/Makefile          |   1 +
 tools/testing/selftests/cpuidle/Makefile  |   7 +
 tools/testing/selftests/cpuidle/cpuidle.c | 479 ++++++++++++++++++++++
 tools/testing/selftests/cpuidle/settings  |   1 +
 4 files changed, 488 insertions(+)
 create mode 100644 tools/testing/selftests/cpuidle/Makefile
 create mode 100644 tools/testing/selftests/cpuidle/cpuidle.c
 create mode 100644 tools/testing/selftests/cpuidle/settings

-- 
2.26.2