Re: Futex hash_bucket lock can break isolation and cause priority inversion on RT

Juri Lelli
2024-10-09T08:29:06Z

Hi Waiman,

On 08/10/24 14:30, Waiman Long wrote:
> On 10/8/24 11:22 AM, Juri Lelli wrote:

...

> > Now, of course by making the latency sensitive application tasks use a
> > higher priority than anything on housekeeping CPUs we could avoid the
> > issue, but the fact that an implicit in-kernel link between otherwise
> > unrelated tasks might cause priority inversion is probably not ideal?
> > Thus this email.
> >
> > Does this report make any sense? If it does, has this issue ever been
> > reported and possibly discussed? I guess it’s kind of a corner case, but
> > I wonder if anybody has suggestions already on how to possibly try to
> > tackle it from a kernel perspective.
>
> Just a question. Is the low latency application using PI futex or the normal
> wait-wake futex? We could use separate set of hash buckets for these
> distinct futex types.

AFAIK it uses normal futexes (or a mix at best). Also I believe it
relies on libraries, so somewhat difficult to tell for certain.

Thanks,
Juri