Re: [RFC v2 PATCH 0/16] Optimize this_cpu_*() ops for non-x86 (ARM64 for this series)

["David Hildenbrand (Arm)" <david@kernel.org>]

On 8/4/26 23:25, Christoph Lameter (Ampere) wrote:
> On Tue, 4 Aug 2026, David Hildenbrand (Arm) wrote:
> 
>>> rather hacky restart logic. It relies on a long preemable and postscript
>>> to each per cpu operations.
> 
>> But I am sure you did you homework and have data to back up your claims. Please
>> share that data, because I am very curious.
> 
> The comparison here is a single instruction on one side and abnd the per
> cpu operation prefixed with a preeamble and another instruction / set of
> instructions on the other side.
> 
> Even if its only one before and one after then we are still looking at 3
> instructions instead of one. This is a frequently used operation in the
> VM.
> 
> You need data to show that one instruction  is faster than 3???

I want you to show that the difference *matters* in *practice* enough that we
should *care* about accepting the additional *complexity*.

No hand-waving about "3 > 1". IIRC people were asking that previously but
received no results back.

So to be clear: claiming that the additional instructions are unacceptable I'd
expect something minimal like: "removing these 3 instructions speeds up
micro-benchmark (a, b, c) by X% and macro-benchmarks (d, e, f) by Y% compared to
Mark's approach".

Maybe there really is a big difference.

> 
>> Mark's solution is the obvious improvement to the problem, doing it just like
>> s390 already does.
> 
> Well there is interest by the S390 folks to move to what we proposed from
> what I can tell.

Heiko said that as reply to v1, bit he's been replying a lot on Mark's patches.
Only Heiko can tell. (on CC)

> 
> 
>> If the proposed solution has a significant benefit over Mark's solution it could
>> be discussed as a next step; and it must be backed with sufficient data to
>> warrant all this complexity here.
> 
> You cannot accept that one instruction is faster than the same
> instruction prefixed with another and something else at its tail?

I am pretty sure that you got exactly what I meant, you likely just don't have
the data to back it up.

Proof me wrong!

-- 
Cheers,

David