Re: [PATCH v4 00/40] lib/find: add atomic find_bit() primitives

Yury Norov
2024-06-20T18:32:25Z

On Thu, Jun 20, 2024 at 11:00:38AM -0700, Linus Torvalds wrote:
> On Thu, 20 Jun 2024 at 10:57, Yury Norov <yury.norov@gmail.com> wrote:
> >
> >
> > The typical lock-protected bit allocation may look like this:
>
> If it looks like this, then nobody cares. Clearly the user in question
> never actually cared about performance, and you SHOULD NOT then say
> "let's optimize this that nobody cares about":.
>
> Yury, I spend an inordinate amount of time just double-checking your
> patches. I ended up having to basically undo one of them just days
> ago.

Is that in master already? I didn't get any email, and I can't find
anything related in the master branch.

> New rule: before you send some optimization, you need to have NUMBERS.

I tried to underline that it's not a performance optimization at my
best. People notice some performance differences, but it's ~3%, no
more.

> Some kind of "look, this code is visible in profiles, so we actually care".

The original motivation comes from a KCSAN report, so it's already
visible in profiles. See [1] in cover letter. This series doesn't fix
that particular issue, but it adds tooling that allow people to search
and acquire bits in bitmaps without firing KCSAN warnings.

This series fixes one real bug in the codebase - see #33, and
simplifies bitmaps usage in many other places. Many people like
it, and acked the patches.

Again, this is NOT a performance series.

Thanks,
Yury

> Because without numbers, I'm just not going to pull anything from you.
> These insane inlines for things that don't matter need to stop.
>
> And if they *DO* matter, you need to show that they matter.
>
> Linus