Re: Candidate Linux ABI for Intel AMX and hypothetical new related features

On Thu, May 20, 2021 at 5:41 PM Thomas Gleixner [off-list ref] wrote:

Len,

On Thu, May 20 2021 at 17:22, Len Brown wrote:

quoted

On Thu, May 20, 2021 at 4:54 PM Thomas Gleixner [off-list ref] wrote:

quoted

quoted

AMX is analogous to the multiplier used by AVX-512.
The architectural state must exist on every CPU, including HT siblings.
Today, the HT siblings share the same execution unit,
and I have no reason to expect that will change.

I'm well aware that HT siblings share the same execution unit for
AVX.

Though AMX is if I remember the discussions two years ago correctly
shared by more than the HT siblings which makes things worse.

I regret that we were unable to get together in the last year to have
an updated discussion.  I think if we had, then we would have saved
a lot of mis-understanding and a lot of email!

So let me emphasize here:

There is one TMUL execution unit per core.
It is shared by the HT siblings within that core.

So the comparison to the AVX-512 multiplier is a good one.

Fine, but that does not at all change the facts that:

  1) It's shared between logical CPUs

  2) It has effects on power/thermal and therefore effects which reach
     outside of the core scope

FWIW, this is true of *every* instruction in the CPU.
Indeed, even when the CPU is executing *no* instructions at all,
the C-state chosen by that CPU has power/thermal impacts on its peers.

Granted, high performance instructions such as AVX-512 and TMUL
are the most extreme case.

  3) Your approach of making it unconditionally available via the
     proposed #NM prevents the OS and subsequently the system admin /
     system designer to implement fine grained control over that
     resource.

     And no, an opt-in approach by providing a non-mandatory
     preallocation prctl does not solve that problem.

I'm perfectly fine with making the explicit allocation (aka opt-in) mandatory,
and enforcing it.

Len Brown, Intel Open Source Technology Center