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

[Jerin Jacob <jerinjacobk@gmail.com>]

On Fri, Oct 9, 2020 at 3:53 PM Burakov, Anatoly
<anatoly.burakov@intel.com> wrote:
>
> On 09-Oct-20 11:17 AM, Thomas Monjalon wrote:
> > 09/10/2020 12:03, 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@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@intel.com> wrote:
> >>>>>>>>> On Thu, Oct 8, 2020 at 6:57 PM Burakov, Anatoly
> >>>>>>>>> <anatoly.burakov@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@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).
> >
> > Obviously an API definition should *never* be "implementation-defined".
>
> If there isn't a meaningful return value, we could either make it a
> void, or return 0/-ENOTSUP so. I'm OK with either as nop is a valid
> result for a UMWAIT, and there are no side-effects to the intrinsic
> itself (it's basically a fancy rte_pause).
>
> >
> >
> >> 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.
> >
> > It is not call "power-x86", so we must assume it could work
> > on any architecture.
>
> #ifdef it is!
>
> >
> >
> >>>> 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.
> >
> > CLDEMOTE is used for optimization, while UMWAIT can be used in a logic,
> > that's why the expectations may be different.
> >
>
> UMWAIT is a best-effort mechanism with no side-effects. It's perfectly
> legal for a UMWAIT to not sleep at all, thus rendering it effectively a
> noop. So i don't think it's all that different.

If a platform does not support UMWAIT in ALL case IMO, no consumer takes this
the path for power saving. So IMO, t is different than CLDEMOTE

>
> --
> Thanks,
> Anatoly