Re: io_uring_prep_openat_direct() and link/drain

[Miklos Szeredi <miklos@szeredi.hu>]

On Sat, 2 Apr 2022 at 03:17, Jens Axboe <axboe@kernel.dk> wrote:
>
> On 4/1/22 10:21 AM, Jens Axboe wrote:
> > On 4/1/22 10:02 AM, Miklos Szeredi wrote:
> >> On Fri, 1 Apr 2022 at 17:36, Jens Axboe <axboe@kernel.dk> wrote:
> >>
> >>> I take it you're continually reusing those slots?
> >>
> >> Yes.
> >>
> >>>  If you have a test
> >>> case that'd be ideal. Agree that it sounds like we just need an
> >>> appropriate breather to allow fput/task_work to run. Or it could be the
> >>> deferral free of the fixed slot.
> >>
> >> Adding a breather could make the worst case latency be large.  I think
> >> doing the fput synchronously would be better in general.
> >
> > fput() isn't sync, it'll just offload to task_work. There are some
> > dependencies there that would need to be checked. But we'll find a way
> > to deal with it.
> >
> >> I test this on an VM with 8G of memory and run the following:
> >>
> >> ./forkbomb 14 &
> >> # wait till 16k processes are forked
> >> for i in `seq 1 100`; do ./procreads u; done
> >>
> >> You can compare performance with plain reads (./procreads p), the
> >> other tests don't work on public kernels.
> >
> > OK, I'll check up on this, but probably won't have time to do so before
> > early next week.
>
> Can you try with this patch? It's not complete yet, there's actually a
> bunch of things we can do to improve the direct descriptor case. But
> this one is easy enough to pull off, and I think it'll fix your OOM
> case. Not a proposed patch, but it'll prove the theory.

Sorry for the delay..

Patch works like charm.

Thanks,
Miklos