Re: io_uring_prep_openat_direct() and link/drain

[Miklos Szeredi <miklos@szeredi.hu>]

On Tue, 5 Apr 2022 at 16:44, Jens Axboe <axboe@kernel.dk> wrote:
>
> On 4/5/22 1:45 AM, Miklos Szeredi wrote:
> > 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.
>
> OK good, then it is the issue I suspected. Thanks for testing!

Tested with v5.18-rc3 and performance seems significantly worse than
with the test patch:

test patch:
        avg     min     max     stdev
real    0.205   0.190   0.266   0.011
user    0.017   0.007   0.029   0.004
sys     0.374   0.336   0.503   0.022

5.18.0-rc3-00016-gb253435746d9:
        avg     min     max     stdev
real    0.725   0.200   18.090  2.279
user    0.019   0.005   0.046   0.006
sys     0.454   0.241   1.022   0.199

Thanks,
Miklos