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

["lihuisong (C)" <lihuisong@huawei.com>]

在 2024/3/22 20:35, Morten Brørup 写道:
>> From: lihuisong (C) [mailto:lihuisong@huawei.com]
>> Sent: Friday, 22 March 2024 09.54
>>
>> +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?
>>
>>>> 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.
>>>>> 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.
> OK... Since microsecond resolution is good enough for ACPI and Linux, you have me convinced that it's also good enough for DPDK (for this specific topic).
>
> Thank you for the detailed reply!
>
>> 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.
> It would defeat the purpose (i.e. not saving sufficient amounts of power) if the CPU cannot enter a deeper idle state.
Yes, it is not good for power.
AFAIS, PM QoS is just to decrease the influence for performance.
Anyway, if we set to zero, system can be into Cstates-0 at least.
>
> Personally, I would think a wake-up latency of up to 10 microseconds should be fine for must purposes.
> Default Linux timerslack is 50 microseconds, so you could also use that value.
How much CPU latency is ok. Maybe, we can give the decision to the 
application.
Linux will collect all these QoS request and use the minimum latency.
what do you think, Morten?
>
>>> 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