linux-kernel - [RFC v4 0/1] Selftest for cpuidle latency measurement

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Message-Id: <20200902114506.45809-1-psampat@linux.ibm.com>
Date:   Wed,  2 Sep 2020 17:15:05 +0530
From:   Pratik Rajesh Sampat <psampat@...ux.ibm.com>
To:     rjw@...ysocki.net, daniel.lezcano@...aro.org,
        srivatsa@...il.mit.edu, shuah@...nel.org, npiggin@...il.com,
        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 v4 0/1] Selftest for cpuidle latency measurement

Changelog v3-->v4:
1. Overhaul in implementation from kernel module to a userspace selftest 
---

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 residency values

To achieve this in the userspace, IPI latencies are calculated by
sending information through pipes and inducing a wakeup, similarly
alarm events are setup for calculate timer based wakeup latencies.

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.

In theory, wakeups induced by IPI and Timers should have similar
wakeup latencies, however in practice there may be deviations which may
need to be captured.

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

--Timeout Latency Test--
--Baseline Timeout Latency measurement: CPU Busy--
Wakeup_src Baseline_delay(ns)
...
 32          972405
 33         1004287
 34          986663
 35          994022
Expected timeout(ns): 10000000
Observed Average timeout diff(ns): 991844

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 | 616 ++++++++++++++++++++++
 tools/testing/selftests/cpuidle/settings  |   1 +
 4 files changed, 625 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