NACK: [SRU][Jammy][lowlatency] Ubuntu: [Config] lowlatency: enhance desktop responsiveness
Dimitri John Ledkov
dimitri.ledkov at canonical.com
Wed Nov 8 08:03:47 UTC 2023
On Mon, 30 Oct 2023 at 19:09, Gerald Yang <gerald.yang at canonical.com> wrote:
>
> BugLink: https://bugs.launchpad.net/bugs/2023007
>
> [Impact]
>
> The lowlatency kernel in Ubuntu is specifically designed to prioritize
> high responsiveness, making it ideal for multimedia environments like
> DAWs and audio processing platforms, as well as soft real-time
> environments.
>
> With the introduction of a real-time kernel, it might be worth
> reconsidering the role of the lowlatency kernel and potentially
> including it as the default kernel in desktop images, focusing on its
> suitability for desktop-oriented usage.
>
> To achieve this, we can enable additional configuration settings and
> make it more focused for a low-latency and highly responsive desktop
> environment.
>
> Optionally (for the future) provide also an additional user-space
> package that would enable specific run-time kernel settings focused at
> certain preset workload profiles (e.g, web navigation, gaming, audio
> processing, etc.).
>
> [Test case]
>
> Use linux-lowlatency in a desktop environment and measure responsiveness
> of interactive applications.
>
> [Fix]
>
> Enable the following additional .config settings to make this kernel
> more suitable for a low-latency desktop kernel:
After careful assessment, this is not suitable for SRU and will
regress the available portfolio of kernels we make available on Focal.
As it is a feature, not a bug.
Whilst all of the above is true, and this will ship in the GA in noble
and in jammy-lowlatency-hwe, it is not an appropriate change for
jammy-ga, nor for focal backports. This is primarily due to lack of an
Ubuntu Realtime Kernel offering on the focal series.
My recommendation is to upgrade to jammy, and opt into either
lowlatency-hwe or realtime kernel in jammy as appropriate. Or wait a
little bit and upgrade to noble.
The jammy GA and focal HWE are otherwise frozen for changes like this,
at the very least since summer 2022.
>
> - CONFIG_NO_HZ_FULL=y: enable access to "Full tickless mode" (shutdown
> clock tick when possible across all the enabled CPUs if they are
> either idle or running 1 task - reduce kernel jitter of running tasks
> due to the periodic clock tick, must be enabled at boot time passing
> `nohz_full=<cpu_list>`)
>
> - CONFIG_RCU_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y: move RCU
> callbacks from softirq context to kthread context (reduce time spent
> in softirqs with preemption disabled to improve the overall system
> responsiveness, at the cost of introducing a potential performance
> penalty, because RCU callbacks are not processed by kernel threads)
>
> [Regression potential]
>
> Enabling all these settings can introduce a potential performance
> regression, but the kernel should result more responsive and make
> it more suitable for a desktop/multimedia/gaming/audio processing
> context.
>
> Signed-off-by: Gerald Yang <gerald.yang at canonical.com>
> ---
> debian.lowlatency/config/annotations | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/debian.lowlatency/config/annotations b/debian.lowlatency/config/annotations
> index 694ae26b6745..3e1556d60809 100644
> --- a/debian.lowlatency/config/annotations
> +++ b/debian.lowlatency/config/annotations
> @@ -15,12 +15,20 @@ CONFIG_HZ_250 note<'Override default HZ used i
> CONFIG_LATENCYTOP policy<{'amd64': 'y', 'arm64': 'y'}>
> CONFIG_LATENCYTOP note<'https://lists.ubuntu.com/archives/kernel-team/2014-July/045006.html, LP#1655986'>
>
> +CONFIG_NO_HZ_FULL policy<{'amd64': 'y', 'arm64': 'y'}>
> +CONFIG_NO_HZ_FULL note<'Enable access to "Full tickless mode" (LP: #2023007)'>
> +
> +CONFIG_NO_HZ_IDLE policy<{'amd64': 'n', 'arm64': 'n'}>
> +CONFIG_NO_HZ_IDLE note<'Disabled in favor of CONFIG_NO_HZ_FULL (LP: #2023007)'>
> +
> CONFIG_PREEMPT policy<{'amd64': 'y', 'arm64': 'y'}>
> CONFIG_PREEMPT note<'Enable fully preemptible kernel'>
>
> CONFIG_PREEMPT_VOLUNTARY policy<{'amd64': 'n', 'arm64': 'n'}>
> CONFIG_PREEMPT_VOLUNTARY note<'Disable voluntary preemption model'>
>
> +CONFIG_RCU_NOCB_CPU policy<{'amd64': 'y', 'arm64': 'y'}>
> +CONFIG_RCU_NOCB_CPU note<'Move RCU callbacks from softirq context to kthread context (LP: #2023007)'>
>
> # ---- Annotations without notes ----
>
> @@ -55,6 +63,8 @@ CONFIG_CEC_PIN policy<{'amd64': 'y', 'arm64': '
> CONFIG_CEC_PIN_ERROR_INJ policy<{'amd64': 'n', 'arm64': 'n'}>
> CONFIG_COMEDI_TESTS_EXAMPLE policy<{'amd64': 'n', 'arm64': 'm'}>
> CONFIG_COMEDI_TESTS_NI_ROUTES policy<{'amd64': 'n', 'arm64': 'm'}>
> +CONFIG_CONTEXT_TRACKING policy<{'amd64': 'y', 'arm64': 'y'}>
> +CONFIG_CONTEXT_TRACKING_FORCE policy<{'amd64': 'n', 'arm64': 'n'}>
> CONFIG_DEBUG_PREEMPT policy<{'amd64': 'n', 'arm64': 'n'}>
> CONFIG_HZ policy<{'amd64': '1000', 'arm64': '1000'}>
> CONFIG_INLINE_READ_LOCK policy<{'arm64': '-'}>
> @@ -89,4 +99,7 @@ CONFIG_PREEMPT_RCU policy<{'amd64': 'y', 'arm64': '
> CONFIG_PREEMPT_TRACER policy<{'amd64': 'n', 'arm64': 'n'}>
> CONFIG_TASKS_RCU policy<{'amd64': 'y', 'arm64': 'y'}>
> CONFIG_TEST_DIV64 policy<{'amd64': 'm', 'arm64': 'm'}>
> +CONFIG_TICK_CPU_ACCOUNTING policy<{'amd64': '-', 'arm64': '-'}>
> CONFIG_UNINLINE_SPIN_UNLOCK policy<{'amd64': 'y', 'arm64': 'y'}>
> +CONFIG_VIRT_CPU_ACCOUNTING policy<{'amd64': 'y', 'arm64': 'y'}>
> +CONFIG_VIRT_CPU_ACCOUNTING_GEN policy<{'amd64': 'y', 'arm64': 'y'}>
> --
> 2.25.1
>
>
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--
okurrr,
Dimitri
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