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[RFC,v3,0/2] CPU-Idle latency selftest framework

Message ID 20230911053620.87973-1-aboorvad@linux.vnet.ibm.com (mailing list archive)
Headers show
Series CPU-Idle latency selftest framework | expand

Message

Aboorva Devarajan Sept. 11, 2023, 5:36 a.m. UTC 
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.com/

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

Comments

Aboorva Devarajan Sept. 25, 2023, 5:06 a.m. UTC | #1 
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

> 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.com/
> 
> 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
>
Aboorva Devarajan Oct. 12, 2023, 4:48 a.m. UTC | #2 
On Mon, 2023-09-25 at 10:36 +0530, Aboorva Devarajan wrote:

Gentle ping to check if there are any feedback or comments on this
patch-set.

Thanks
Aboorva

> 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
> 
> > 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.com/
> > 
> > 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
> >