Re: [PATCH 3/4] rust: id_pool: add contiguous area allocation

[Yury Norov <ynorov@nvidia.com>]

On Wed, Jul 08, 2026 at 07:33:16AM +0200, Greg KH wrote:

...

> > I asked exactly the same question when Alice and Burak added wrappers
> > for bitmaps to implement their ID pool. This is the answer:
> > 
> >   An alternative route of vendoring an existing Rust bitmap package was
> >   considered but suboptimal overall. Reusing the C implementation is
> >   preferable for a basic data structure like bitmaps. It enables Rust
> >   code to be a lot more similar and predictable with respect to C code
> >   that uses the same data structures and enables the use of code that
> >   has been tried-and-tested in the kernel, with the same performance
> >   characteristics whenever possible.
> > 
> > And now it's in a commit message: 11eca92a2caeb
> > 
> > They measured the affect of their wrapper on performance, and it appears
> > to be ~5%. See lib/find_bit_benchmark_rust.rs.
> 
> You are comparing the C vs. Rust data structures here, which is not what
> I am proposing.
> 
> Also, is this code being used on a "hot path" like the binder stuff is?
> 
> > I didn't see any side-to-side comparison between any native Rust API vs
> > imported C bitmaps. I'm sure, I asked for that, and I still believe
> > it's the important piece of data to avoid this back-and-forth type of
> > discussions. So, Alice, Burak or anybody...
> 
> Again, I'm not talking about Rust API vs. imported C bitmaps, I'm asking
> to use the C structures like maple-tree and idr instead of open-coding
> logic around the bitmap code.

I understand your point. I asked both questions: are they sure that bitmap
is the most optimal data structure for the ID pool, and if so, why not use
the built-in Rust bitmaps? The answer was: yes, it's the most optimal, and
using built-in bitmaps is suboptimal overall.

Burak, can you share more details please?

> > > > > Why isn't the built-in idr library being used here instead of rolling
> > > > > your own data structure?
> > 
> > Now having more context, the ID pool's primary goal is to allocate
> > individual IDs, which naturally lays on find_bit() API in C. The
> > native Rust alternative is considered and found 'suboptimal overall'.
> 
> Allocating IDs is a probe() thing, which can be as slow as it wants,
> right?  Or is this some other hot-path where performance matters?  The
> patch was not very specific as to the tradeoffs needed.

I am a bitmaps maintainer, and I want them in Rust to perform equally well.
The ID pool case was just the first user. Even if bitmap performance is
not a critical path for ID pool, my role is to make sure that bitmap in
Rust is implemented well, including the performance part.

Said that, I recall that bitmaps performance was important for the ID
pool, and Alice and Burak spent some time optimizing the ID allocation
path. Particularly, added a logic to avoid allocation of small bitmaps.

Thanks,
Yury