On Mon, 2023-09-11 at 11:06 +0530, Aboorva Devarajan wrote:

CC'ing CPUidle lists and maintainers,

Patch Summary:

The patchset introduces a kernel module and userspace driver designed
for estimating the wakeup latency experienced when waking up from
various CPU idle states. It primarily measures latencies related to two
types of events: Inter-Processor Interrupts (IPIs) and Timers.

Background:

Initially, these patches were introduced as a generic self-test.
However, it was later discovered that Intel platforms incorporate
timer-based wakeup optimizations. These optimizations allow CPUs to
perform a pre-wakeup, which limits the effectiveness of latency
observation in certain scenarios because it only measures the optimized
wakeup latency [1]. 

Therefore, in this RFC, the self-test is specifically integrated into
PowerPC, as it has been tested and used in PowerPC so far.

Another proposal is to introduce these patches as a generic cpuilde IPI
and timer wake-up test. While this method may not give us an exact
measurement of latency variations at the hardware level, it can still
help us assess this metric from a software observability standpoint.

Looking forward to hearing what you think and any suggestions you may
have regarding this. Thanks.

[1] 
https://lore.kernel.org/linux-pm/20200914174625.GB25628@in.ibm.com/T/#m5c004b9b1a918f669e91b3d0f33e2e3500923234 (local)

Changelog: v2 -> v3

* Minimal code refactoring
* Rebased on v6.6-rc1

RFC v1: 
https://lore.kernel.org/all/20210611124154.56427-1-psampat@linux.ibm.com/ (local)

RFC v2:
https://lore.kernel.org/all/20230828061530.126588-2-aboorvad@linux.vnet.ibm.com/ (local)

Other related RFC:
https://lore.kernel.org/all/20210430082804.38018-1-psampat@linux.ibm.com/ (local)

Userspace selftest:
https://lkml.org/lkml/2020/9/2/356

----

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 be 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/powerpc/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 is as follows:

# --IPI Latency Test---
# Baseline Avg IPI latency(ns): 2720
# Observed Avg IPI latency(ns) - State snooze: 2565
# Observed Avg IPI latency(ns) - State stop0_lite: 3856
# Observed Avg IPI latency(ns) - State stop0: 3670
# Observed Avg IPI latency(ns) - State stop1: 3872
# Observed Avg IPI latency(ns) - State stop2: 17421
# Observed Avg IPI latency(ns) - State stop4: 1003922
# Observed Avg IPI latency(ns) - State stop5: 1058870
#
# --Timeout Latency Test--
# Baseline Avg timeout diff(ns): 1435
# Observed Avg timeout diff(ns) - State snooze: 1709
# Observed Avg timeout diff(ns) - State stop0_lite: 2028
# Observed Avg timeout diff(ns) - State stop0: 1954
# Observed Avg timeout diff(ns) - State stop1: 1895
# Observed Avg timeout diff(ns) - State stop2: 14556
# Observed Avg timeout diff(ns) - State stop4: 873988
# Observed Avg timeout diff(ns) - State stop5: 959137

Aboorva Devarajan (2):
  powerpc/cpuidle: cpuidle wakeup latency based on IPI and timer
events
  powerpc/selftest: Add support for cpuidle latency measurement

 arch/powerpc/Kconfig.debug                    |  10 +
 arch/powerpc/kernel/Makefile                  |   1 +
 arch/powerpc/kernel/test_cpuidle_latency.c    | 154 ++++++
 tools/testing/selftests/powerpc/Makefile      |   1 +
 .../powerpc/cpuidle_latency/.gitignore        |   2 +
 .../powerpc/cpuidle_latency/Makefile          |   6 +
 .../cpuidle_latency/cpuidle_latency.sh        | 443
++++++++++++++++++
 .../powerpc/cpuidle_latency/settings          |   1 +
 8 files changed, 618 insertions(+)
 create mode 100644 arch/powerpc/kernel/test_cpuidle_latency.c
 create mode 100644
tools/testing/selftests/powerpc/cpuidle_latency/.gitignore
 create mode 100644
tools/testing/selftests/powerpc/cpuidle_latency/Makefile
 create mode 100755
tools/testing/selftests/powerpc/cpuidle_latency/cpuidle_latency.sh
 create mode 100644
tools/testing/selftests/powerpc/cpuidle_latency/settings