Re: [RFC PATCH 3/3] treewide: pipe: Convert all references to pipe->{head,tail,max_usage,ring_size} to unsigned short

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, 6 Mar 2025 at 05:33, K Prateek Nayak <kprateek.nayak@amd.com> wrote:
> >>
> >> I dunno... but if we do this, perhaps we should
> >> s/unsigned int/pipe_index_t instead?
> >>
> >> At least this would be more grep friendly.
>
> Ack. I'll leave the typedef untouched and convert these to use
> pipe_index_t. This was an experiment so see if anything breaks with u16
> conversion just to get more testing on that scenario. As Rasmus
> mentioned, leaving the head and tail as u32 on 64bit will lead to
> better code generation.

Yes, I was going to say the same - please don't change to 'unsigned short'.

Judicious use of 'pipe_index_t' may be a good idea, but as I fixed
some issues Rasmus found, I was also looking at the generated code,
and on at least x86 where 16-bit generates extra instructions and
prefixes, it seems marginally better to treat the values as 32-bit,
and then only do the compares in 16 bits.

That only causes a few "movzwl" instructions (at load time), and then
the occasional "cmpw" (empty check) and "movw" (store) etc.

But I only did a very quick "let's look at a few cases of x86-64 also
using a 16-bit pipe_index_t".

So for testing purposes your patch looks fine, but not as something to apply.

If anything, I think we should actively try to remove as many direct
accesses to these pipe fields as humanly possible. As Oleg said, a lot
of them should just be cleaned up to use the helpers we already have.

Rasmus found a few cases of that already, like that FIONREAD case
where it was just doing a lot of open-coding of things that shouldn't
be open-coded.

I've fixed the two cases he pointed at up as obvious bugs, but it
would be good to see where else issues like this might lurk.

                 Linus