[SRU][Jammy][lowlatency] Ubuntu: [Config] lowlatency: enhance desktop responsiveness
Andrea Righi
andrea.righi at canonical.com
Tue Oct 31 06:28:52 UTC 2023
On Tue, Oct 31, 2023 at 12:57:39AM +0800, Gerald Yang 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:
>
> - 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>
It's worth mentioning that we already applied these configs to the
lowlatency kernels in mantic and in general I don't see any downside if
we apply this.
However, I'm not sure if a change like this is proper SRU material, we
are basically saying "now lowlatency is a desktop kernel everywhere".
Certain workloads may experience regressions (especially with the
RCU_NOCB_CPU), while NO_HZ_FULL is a lot safer (since it can be
enabled/disabled via boot options).
We also need to keep in mind that Jammy will get these changes with the
lowlatency-hwe-6.5 kernel, so I'm not still 100% convinced to backport
this and apply it everywhere... opinions?
-Andrea
> ---
> 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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