Re: [PATCH 0/2] introduce PM QoS interface

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

On Fri, Mar 22, 2024 at 04:54:01PM +0800, lihuisong (C) wrote:
> +Tyler, +Alan, +Wei, +Long for asking this similar feature on Windows.
> 
> 在 2024/3/21 21:30, Morten Brørup 写道:
> >>From: lihuisong (C) [mailto:lihuisong@huawei.com]
> >>Sent: Thursday, 21 March 2024 04.04
> >>
> >>Hi Moren,
> >>
> >>Thanks for your revew.
> >>
> >>在 2024/3/20 22:05, Morten Brørup 写道:
> >>>>From: Huisong Li [mailto:lihuisong@huawei.com]
> >>>>Sent: Wednesday, 20 March 2024 11.55
> >>>>
> >>>>The system-wide CPU latency QoS limit has a positive impact on the idle
> >>>>state selection in cpuidle governor.
> >>>>
> >>>>Linux creates a cpu_dma_latency device under '/dev' directory to obtain the
> >>>>CPU latency QoS limit on system and send the QoS request for userspace.
> >>>>Please see the PM QoS framework in the following link:
> >>>>https://docs.kernel.org/power/pm_qos_interface.html?highlight=qos
> >>>>This feature is supported by kernel-v2.6.25.
> >>>>
> >>>>The deeper the idle state, the lower the power consumption, but the longer
> >>>>the resume time. Some service are delay sensitive and very except the low
> >>>>resume time, like interrupt packet receiving mode.
> >>>>
> >>>>So this series introduce PM QoS interface.
> >>>This looks like a 1:1 wrapper for a Linux kernel feature.
> >>right
> >>>Does Windows or BSD offer something similar?
> >>How do we know Windows or BSD support this similar feature?
> >Ask Windows experts or research using Google.
> I download freebsd source code, I didn't find this similar feature.
> They don't even support cpuidle feature(this QoS feature affects cpuilde.).
> I don't find any useful about this on Windows from google.
> 
> 
> @Tyler, @Alan, @Wei and @Long
> 
> Do you know windows support that userspace read and send CPU latency
> which has an impact on deep level of CPU idle?

it is unlikely you'll find an api that let's you manage things in terms
of raw latency values as the linux knobs here do. windows more often employs
policy centric schemes to permit the system to abstract implementation detail.

powercfg is probably the closest thing you can use to tune the same
things on windows. where you select e.g. the 'performance' scheme but it
won't allow you to pick specific latency numbers.

https://learn.microsoft.com/en-us/windows-hardware/design/device-experiences/powercfg-command-line-options

> 
> >>The DPDK power lib just work on Linux according to the meson.build under
> >>lib/power.
> >>If they support this features, they can open it.
> >The DPDK power lib currently only works on Linux, yes.
> >But its API should still be designed to be platform agnostic, so the functions can be implemented on other platforms in the future.
> >
> >DPDK is on track to work across multiple platforms, including Windows.
> >We must always consider other platforms, and not design DPDK APIs as if they are for Linux/BSD only.
> totally understand you.

since lib/power isn't built for windows at this time i don't think it's
appropriate to constrain your innovation. i do appreciate the engagement
though and would just offer general guidance that if you can design your
api with some kind of abstraction in mind that would be great and by all
means if you can figure out how to wrangle powercfg /Qh into satisfying the
api in a policy centric way it might be kind of nice.

i'll let other windows experts chime in here if they choose.

thanks!

> >
> >>>Furthermore, any high-res timing should use nanoseconds, not microseconds or
> >>milliseconds.
> >>>I realize that the Linux kernel only uses microseconds for these APIs, but
> >>the DPDK API should use nanoseconds.
> >>Nanoseconds is more precise, it's good.
> >>But DPDK API how use nanoseconds as you said the the Linux kernel only
> >>uses microseconds for these APIs.
> >>Kernel interface just know an integer value with microseconds unit.
> >One solution is to expose nanoseconds in the DPDK API, and in the Linux specific implementation convert from/to microseconds.
> If so, we have to modify the implementation interface on Linux. This
> change the input/output unit about the interface.
> And DPDK also has to do this based on kernel version. It is not good.
> The cpuidle governor select which idle state based on the worst-case
> latency of idle state.
> These the worst-case latency of Cstate reported by ACPI table is in
> microseconds as the section 8.4.1.1. _CST (C States) and 8.4.3.3.
> _LPI (Low Power Idle States) in ACPI spec [1].
> So it is probably not meaning to change this interface implementation.
> 
> For the case need PM QoS in DPDK, I think, it is better to set cpu
> latency to zero to prevent service thread from the deeper the idle
> state.
> >You might also want to add a note to the in-line documentation of the relevant functions that the Linux implementation only uses microsecond resolution.
> >
> [1] https://uefi.org/specs/ACPI/6.5/08_Processor_Configuration_and_Control.html