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

[Thomas Monjalon <thomas@monjalon.net>]

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".


> 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.


> >> 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.