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/#m5c004...
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/
RFC v2: https://lore.kernel.org/all/20230828061530.126588-2-aboorvad@linux.vnet.ibm....
Other related RFC: https://lore.kernel.org/all/20210430082804.38018-1-psampat@linux.ibm.com/
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