[dpdk-dev] [PATCH v4 02/10] eal: add power management intrinsics

[Burakov, Anatoly]

On 09-Oct-20 10:54 AM, Jerin Jacob wrote:
> On Fri, Oct 9, 2020 at 3:10 PM Burakov, Anatoly
> <anatoly.burakov at intel.com> wrote:
>>
>> On 09-Oct-20 10:29 AM, Thomas Monjalon wrote:
>>> 09/10/2020 11:25, Burakov, Anatoly:
>>>> On 09-Oct-20 6:42 AM, Jerin Jacob wrote:
>>>>> On Thu, Oct 8, 2020 at 10:38 PM Ananyev, Konstantin
>>>>> <konstantin.ananyev at intel.com> wrote:
>>>>>>
>>>>>>>
>>>>>>> On Thu, Oct 8, 2020 at 6:57 PM Burakov, Anatoly
>>>>>>> <anatoly.burakov at intel.com> wrote:
>>>>>>>>
>>>>>>>> On 08-Oct-20 9:44 AM, Jerin Jacob wrote:
>>>>>>>>> On Thu, Oct 8, 2020 at 2:04 PM Thomas Monjalon <thomas at monjalon.net> wrote:
>>>>>>>>>>
>>>>>>>>>>> Add two new power management intrinsics, and provide an implementation
>>>>>>>>>>> in eal/x86 based on UMONITOR/UMWAIT instructions. The instructions
>>>>>>>>>>> are implemented as raw byte opcodes because there is not yet widespread
>>>>>>>>>>> compiler support for these instructions.
>>>>>>>>>>>
>>>>>>>>>>> The power management instructions provide an architecture-specific
>>>>>>>>>>> function to either wait until a specified TSC timestamp is reached, or
>>>>>>>>>>> optionally wait until either a TSC timestamp is reached or a memory
>>>>>>>>>>> location is written to. The monitor function also provides an optional
>>>>>>>>>>> comparison, to avoid sleeping when the expected write has already
>>>>>>>>>>> happened, and no more writes are expected.
>>>>>>>>>>>
>>>>>>>>>>> For more details, Please reference Intel SDM Volume 2.
>>>>>>>>>>
>>>>>>>>>> I really would like to see feedbacks from other arch maintainers.
>>>>>>>>>> Unfortunately they were not Cc'ed.
>>>>>>>>>
>>>>>>>>> Shared the feedback from the arm64 perspective here. Yet to get a reply on this.
>>>>>>>>> http://mails.dpdk.org/archives/dev/2020-September/181646.html
>>>>>>>>>
>>>>>>>>>> Also please mark the new functions as experimental.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>
>>>>>>>> Hi Jerin,
>>>>>>>
>>>>>>> Hi Anatoly,
>>>>>>>
>>>>>>>>
>>>>>>>>     > IMO, We must introduce some arch feature-capability _get_ scheme to tell
>>>>>>>>     > the consumer of this API is only supported on x86. Probably as
>>>>>>>> functions[1]
>>>>>>>>     > or macro flags scheme and have a stub for the other architectures as the
>>>>>>>>     > API marked as generic ie rte_power_* not rte_x86_..
>>>>>>>>     >
>>>>>>>>     > This will help the consumer to create workers based on the
>>>>>>>> instruction features
>>>>>>>>     > which can NOT be abstracted as a generic feature across the
>>>>>>>> architectures.
>>>>>>>>
>>>>>>>> I'm not entirely sure what you mean by that.
>>>>>>>>
>>>>>>>> I mean, yes, we should have added stubs for other architectures, and we
>>>>>>>> will add those in future revisions, but what does your proposed runtime
>>>>>>>> check accomplish that cannot currently be done with CPUID flags?
>>>>>>>
>>>>>>>
>>>>>>> RTE_CPUFLAG_WAITPKG  flag definition is not available in other architectures.
>>>>>>> i.e RTE_CPUFLAG_WAITPKG defined in lib/librte_eal/x86/include/rte_cpuflags.h
>>>>>>> and it is used in http://patches.dpdk.org/patch/79540/ as generic API.
>>>>>>> I doubt http://patches.dpdk.org/patch/79540/  would compile on non-x86.
>>>>>>
>>>>>>
>>>>>> I am agree with Jerin, that we need some generic way to
>>>>>> figure-out does platform supports power_monitor() or not.
>>>>>> Though not sure do we need to create a new feature-get framework here...
>>>>>
>>>>> That's works too. Some means of generic probing is fine. Following
>>>>> schemed needs
>>>>> more documentation on that usage, as, it is not straight forward compare to
>>>>> feature-get framework. Also, on the other thread, we are adding the
>>>>> new instructions like
>>>>> demote cacheline etc, maybe if the user wants to KNOW if the arch
>>>>> supports it then
>>>>> the feature-get framework is good.
>>>>> If we think, there is no other usecase for generic arch feature-get
>>>>> framework then
>>>>> we can keep the below scheme else generic arch feature is better for
>>>>> more forward
>>>>> looking use cases.
>>>>>
>>>>>> Might be just something like:
>>>>>>     rte_power_monitor(...) == -ENOTSUP
>>>>>> be enough indication for that?
>>>>>> So user can just do:
>>>>>> if (rte_power_monitor(NULL, 0, 0, 0, 0) == -ENOTSUP) {
>>>>>>            /* not supported  path */
>>>>>> }
>>>>>>
>>>>>> To check is that feature supported or not.
>>>>>
>>>>>
>>>>
>>>> Looking at CLDEMOTE patches, CLDEMOTE is a noop on other archs. I think
>>>> we can safely make this intrinsic as a noop on other archs as well, as
>>>> it's functionally identical to waking up immediately.
>>>>
>>>> If we're not creating this for CLDEMOTE, we don't need it here as well.
>>>> If we do need it for this, then we arguably need it for CLDEMOTE too.
>>>
>>> Sorry I don't understand what you mean, too many "it" and "this" :)
>>>
>>
>> Sorry, i meant "the generic feature-get framework". CLDEMOTE doesn't
>> exist on other archs, this doesn't too, so it's a fairly similar
>> situation. Stubbing UMWAIT with a noop is a valid approach because it's
>> equivalent to sleeping and then immediately waking up (which can happen
>> for a host of reasons unrelated to the code itself).
> 
> If we are keeping the following return in the public API then it can not be NOP
> + * @return
> + *   - 1 if wakeup was due to TSC timeout expiration.
> + *   - 0 if wakeup was due to memory write or other reasons.
> + */
> 

In the generic header, it is specified that return value is 
implementation-defined (i.e. arch-specific). I guess we could remove 
that and set return value to either 0 or -ENOTSUP if that would resolve 
the issue?

> Also, we need to fix compilation issue if any with
> http://patches.dpdk.org/patch/79540/
> as it has direct reference to if
> (!rte_cpu_get_flag_enabled(RTE_CPUFLAG_WAITPKG)) {
> Either we need to add -ENOTSUP return or generic feature-get framework.

IIRC power library isn't compiled on anything other than x86, so this 
code wouldn't get compiled.

> 
> 
>>
>> I'm not against a generic feature-get framework, i'm just pointing out
>> that if this is what's preventing the merge, it should prevent the merge
>> of CLDEMOTE as well, yet Jerin has acked that one and has explicitly
>> stated that he's OK with leaving CLDEMOTE as a noop on other architectures.
>>
>> --
>> Thanks,
>> Anatoly


-- 
Thanks,
Anatoly