RE: [PATCH] config/x86: config support for AMD EPYC processors

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> > > > > > >> From: Tummala, Sivaprasad <Sivaprasad.Tummala@amd.com>
> > > > > > >>> From: David Marchand <david.marchand@redhat.com>
> > > > > > >>> On Mon, Sep 25, 2023 at 5:11 PM Sivaprasad Tummala
> > > > > > >>>> From: Sivaprasad Tummala <Sivaprasad.Tummala@amd.com>
> > > > > > >>>>
> > > > > > >>>> By default, max lcores are limited to 128 for x86
> > platforms.
> > > > > > >>>> On AMD EPYC processors, this limit needs to be increased
> > to
> > > > > > leverage
> > > > > > >>>> all the cores.
> > > > > > >>>>
> > > > > > >>>> The patch adjusts the limit specifically for native
> > > > compilation on
> > > > > > >>>> AMD EPYC CPUs.
> > > > > > >>>>
> > > > > > >>>> Signed-off-by: Sivaprasad Tummala
> > <Sivaprasad.Tummala@amd.com>
> > > > > > >>>
> > > > > > >>> This patch is a revamp of
> > > > > > >>>
> > > > > > >>
> > > > > >
> > > >
> > http://inbox.dpdk.org/dev/BY5PR12MB3681C3FC6676BC03F0B42CCC96789@BY5PR
> > > > > > >>> 12MB3681.namprd12.prod.outlook.com/
> > > > > > >>> for which a discussion at techboard is supposed to have
> > taken
> > > > place.
> > > > > > >>> But I didn't find a trace of it.
> > > > > > >>>
> > > > > > >>> One option that had been discussed in the previous thread
> > was
> > > > to
> > > > > > >>> increase the max number of cores for x86.
> > > > > > >>> I am unclear if this option has been properly
> > > > evaluated/debatted.
> > > > >
> > > > > Here are the minutes from the previous techboard discussions:
> > > > > [1]: http://inbox.dpdk.org/dev/YZ43U36bFWHYClAi@platinum/
> > > > > [2]:
> > http://inbox.dpdk.org/dev/20211202112506.68acaa1a@hermes.local/
> > > > >
> > > > > AFAIK, there has been no progress with dynamic max_lcores, so I
> > guess
> > > > the techboard's conclusion still stands:
> > > > >
> > > > > There is no identified use-case where a single application
> > requires
> > > > more than 128 lcores. If a case a use-case exists for a single
> > > > application that uses more than 128 lcores, the TB is ok to update
> > the
> > > > default config value.
> > > > >
> > > > > > >>>
> > > > > > >>> Can the topic be brought again at techboard?
> > > > > > >>
> > > > > > >> Hi David,
> > > > > > >>
> > > > > > >> The patch is intended to detect AMD platforms and enable all
> > CPU
> > > > > > cores by default
> > > > > > >> on native builds.
> > > > >
> > > > > This is done on native ARM builds, so why not on native X86
> > builds
> > > > too?
> > > > >
> > > > > > >>
> > > > > > >> As an optimization for memory footprint, users can override
> > this
> > > > by
> > > > > > specifying "-
> > > > > > >> Dmax_lcores" option based on DPDK lcores required for their
> > > > usecases.
> > > > > > >>
> > > > > > >> Sure, will request to add this topic for discussion at
> > > > techboard.
> > > >
> > > > This is the summary of the techboard meeting:
> > > > (see https://mails.dpdk.org/archives/dev/2023-October/279672.html)
> > > >
> > > > - There is some asks for more than 128 worker cores
> > > > - Discussion about generally increasing the default max core count
> > and
> > > > trade-offs with memory consumption but this is longer term issue
> > >
> > > The distros are currently satisfied with the 128 cores default, so
> > the decision here was: Leave the 128 cores default as is, for now.
> > >
> > > Any long term improvements regarding memory consumption of many-core
> > systems are not relevant for this patch.
> > >
> > > > - Acceptance for the direction of this patch in the short term
> > >
> > > With the twist that it must work for cross compile. It is the
> > properties of the target CPU that matter, not the properties of the
> > host
> > > CPU. (Although the build may be "native", i.e. the target CPU is the
> > same as the host CPU, it is still the target CPU that matters.)
> > >
> > > > - Details of whether it should be for EPYC only or x86 to be
> > figured
> > > > out
> > > > on mailing list
> > >
> > > I think this is obvious...
> > >
> > > ARM already provides ARM CPU specific optimizations.
> > > AMD should be allowed to provide AMD CPU specific optimizations too.
> > > Intel can also provide Intel CPU specific optimizations.
> >
> > I suppose no-one stopping AMD/Intel/ARM to provide their CPU specific
> > optimizations.
> > Though as end-user, my preference would be to have one generic build
> > (machine=default) that would work ok
> > on all cpus for given architecture (let say x86) instead of
> > maintaining/testing dozens of different flavors.
> 
> Agree. Machine specific builds should be explicitly specified. I consider "native" a variant of explicitly specifying the target machine.
> 
> > I suppose for 23.11 we have not much choice but accept that patch as it
> > is.
> 
> No. They agreed in the techboard meeting to rework it for cross compile.

Ah, yes, cross-builds, nearly forgot about them.
I suppose yes, you are right, it needs to be supported for completeness.
 
> 
> > Though I think in future (24.11?) it would be ideal to make
> > RTE_MAX_LCORE a runtime parameter and
> > remove it from public API structs.
> 
> It is probably more difficult than it sounds to fully remove RTE_MAX_LCORE.

I am not saying it is not an easy one, otherwise probably it will be already done.
 
> However, if we could make it mostly a runtime parameter, and only keep RTE_MAX_LCORE in arrays of small structures, we could
> increase it to some very large (but still sane) value, like 4096 or whatever number of CPU cores can be found in the largest system out
> there.
> >
> > >
> > > And if some of these optimizations are rooted in the same criteria,
> > they should be shared across the relevant CPU architectures. We
> > > follow this principle in the source code files, and the principle
> > also applies to the build files.
> > >
> > > >
> > > > So now let's figure out the details please.
> > > > Suggestions?
> > >
> > > Suggestions provided inline above. :-)