<div dir="ltr">Thanks Andrea for the information<div>In this case, I'd like to confirm if this will be merged into 5.15 lowlatency-hwe kernel on Focal? not GA on Jammy</div><div>So I can provide some feedbacks to the customer</div><div> </div><div>Thanks,</div><div>Gerald</div></div> <div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Oct 31, 2023 at 10:57 PM Andrea Righi <<a href="mailto:andrea.righi@canonical.com">andrea.righi@canonical.com</a>> wrote: </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">On Tue, Oct 31, 2023 at 09:48:55PM +0800, Gerald Yang wrote: > Hi Dimitri, > > Thanks for checking this! > And yes, that would be very helpful to enable it in focal lowlatency-hwe > and jammy lowlatency-hwe kernels, > at least users/customers could have a way to use this feature. > > Another question I'd like to ask is roughly when will 6.5 lowlatency-hwe > kernel be landed in Jammy? Should I also submit this to 6.2 lowlatency > kernel? We are working on hwe-6.5/lowlatency-hwe-6.5 right now, we are in the process of fixing the usual dkms breakage, reviewing test results, etc... at this point I think we need to wait after the sprint to have the 6.5's out in jammy. -Andrea > > Thanks, > Gerald > > > On Tue, Oct 31, 2023 at 8:36 PM Dimitri John Ledkov < > <a href="mailto:dimitri.ledkov@canonical.com" target="_blank">dimitri.ledkov@canonical.com</a>> wrote: > > > Hi, > > > > On Tue, 31 Oct 2023 at 10:54, Gerald Yang <<a href="mailto:gerald.yang@canonical.com" target="_blank">gerald.yang@canonical.com</a>> > > wrote: > > > > > > Hi Andrea, > > > > > > Thanks for the feedback! please let me explain this in more detail > > > > > > I sent out this SRU because some of our customers need to use NO_HZ_FULL. > > > In their scenario, the CPU time is pretty sensitive, and must avoid as > > much noise as possible to keep > > > most CPUs doing only one single task. > > > I found some CPU steal time happen on their VM because RCU callback > > softirqs are still triggered on CPUs > > > with isolcpus, nohz_full and rcu_nocbs configured, and lead me to find > > NO_HZ_FULL is not built into generic kernel. > > > > > > I think the use case for NO_HZ_FULL should be different than generic > > kernel, e.g. performance vs low latency > > > > > > On generic kernel, we also did some tests with NO_HZ_FULL built-in here: > > > <a href="https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1919154" rel="noreferrer" target="_blank">https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1919154</a> > > > > > > The results show performance degradation on AMD EPYC series machines only > > > when NO_HZ_FULL is built-in but not enabled, but this doesn't happen on > > Intel and arm64 machines. > > > > > > Because one of the customer is still on Focal, if it's possible to merge > > this into 5.15 kernel, it would be more convenient > > > for them to use HWE lowlatency kernel. > > > And if there are users need to use NO_HZ_FULL on Focal, lowlatency > > kernel could be a proper choice for them, > > > because so far there is no kernel with NO_HZ_FULL built-in on Focal. > > > > I am ok having this enabled in focal:lowlatency-hwe-5.15 and > > jammy:lowlatency-hwe-6.5 > > > > but not change anything in focal:lowlatency (5.4 / ga) > > jammy:lowlatency (5.15 / ga). > > > > Would that help you? As an exception to our policy. I really feel > > uncomfortable changing default GA flavours, but risk levels of making > > hwe kernels more like the future sounds safer. > > > > > > > > Thanks, > > > Gerald > > > > > > On Tue, Oct 31, 2023 at 2:28 PM Andrea Righi <<a href="mailto:andrea.righi@canonical.com" target="_blank">andrea.righi@canonical.com</a>> > > wrote: > > >> > > >> On Tue, Oct 31, 2023 at 12:57:39AM +0800, Gerald Yang wrote: > > >> > BugLink: <a href="https://bugs.launchpad.net/bugs/2023007" rel="noreferrer" target="_blank">https://bugs.launchpad.net/bugs/2023007</a> > > >> > > > >> > [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 <<a href="mailto:gerald.yang@canonical.com" target="_blank">gerald.yang@canonical.com</a>> > > >> > > >> 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<' > > <a href="https://lists.ubuntu.com/archives/kernel-team/2014-July/045006.html" rel="noreferrer" target="_blank">https://lists.ubuntu.com/archives/kernel-team/2014-July/045006.html</a>, > > 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 > > >> > > > >> > > > >> > -- > > >> > kernel-team mailing list > > >> > <a href="mailto:kernel-team@lists.ubuntu.com" target="_blank">kernel-team@lists.ubuntu.com</a> > > >> > <a href="https://lists.ubuntu.com/mailman/listinfo/kernel-team" rel="noreferrer" target="_blank">https://lists.ubuntu.com/mailman/listinfo/kernel-team</a> > > > > > > -- > > > kernel-team mailing list > > > <a href="mailto:kernel-team@lists.ubuntu.com" target="_blank">kernel-team@lists.ubuntu.com</a> > > > <a href="https://lists.ubuntu.com/mailman/listinfo/kernel-team" rel="noreferrer" target="_blank">https://lists.ubuntu.com/mailman/listinfo/kernel-team</a> > > > > > > > > -- > > okurrr, > > > > Dimitri > > </blockquote></div>