Re: [PATCH v12 2/2] fuse: add default_request_timeout and max_request_timeout sysctls

Joanne Koong
2025-01-23T18:33:06Z

On Thu, Jan 23, 2025 at 10:06 AM Bernd Schubert
<bernd.schubert@fastmail.fm> wrote:
>
>
>
> On 1/23/25 18:48, Joanne Koong wrote:
> > On Thu, Jan 23, 2025 at 9:19 AM Bernd Schubert
> > <bernd.schubert@fastmail.fm> wrote:
> >>
> >> Hi Joanne,
> >>
> >>>>> Thanks, applied and pushed with some cleanups including Luis's clamp idea.
> >>>>
> >>>> Hi Miklos,
> >>>>
> >>>> I don't think the timeouts do work with io-uring yet, I'm not sure
> >>>> yet if I have time to work on that today or tomorrow (on something
> >>>> else right now, I can try, but no promises).
> >>>
> >>> Hi Bernd,
> >>>
> >>> What are your thoughts on what is missing on the io-uring side for
> >>> timeouts? If a request times out, it will abort the connection and
> >>> AFAICT, the abort logic should already be fine for io-uring, as users
> >>> can currently abort the connection through the sysfs interface and
> >>> there's no internal difference in aborting through sysfs vs timeouts.
> >>>
> >>
> >> in fuse_check_timeout() it iterates over each fud and then fpq.
> >> In dev_uring.c fpq is is per queue but unrelated to fud. In current
> >> fuse-io-uring fud is not cloned anymore - using fud won't work.
> >> And Requests are also not queued at all on the other list
> >> fuse_check_timeout() is currently checking.
> >
> > In the io-uring case, there still can be fuds and their associated
> > fpqs given that /dev/fuse can be used still, no? So wouldn't the
> > io-uring case still need this logic in fuse_check_timeout() for
> > checking requests going through /dev/fuse?
>
> Yes, these need to be additionally checked.
>
> >
> >>
> >> Also, with a ring per core, maybe better to use
> >> a per queue check that is core bound? I.e. zero locking overhead?
> >
> > How do you envision a queue check that bypasses grabbing the
> > queue->lock? The timeout handler could be triggered on any core, so
> > I'm not seeing how it could be core bound.
>
> I don't want to bypass it, but maybe each queue could have its own
> workq and timeout checker? And then use queue_delayed_work_on()?
>
>
> >
> >> And I think we can also avoid iterating over hash lists (queue->fpq),
> >> but can use the 'ent_in_userspace' list.
> >>
> >> We need to iterate over these other entry queues anyway:
> >>
> >> ent_w_req_queue
> >> fuse_req_bg_queue
> >> ent_commit_queue
> >>
> >
> > Why do we need to iterate through the ent lists (ent_w_req_queue and
> > ent_commit_queue)? AFAICT, in io-uring a request is either on the
> > fuse_req_queue/fuse_req_bg_queue or on the fpq->processing list. Even
> > when an entry has been queued to ent_w_req_queue or ent_commit_queue,
> > the request itself is still queued on
> > fuse_req_queue/fuse_req_bg_queue/fpq->processing. I'm not sure I
> > understand why we still need to look at the ent lists?
>
> Yeah you are right, we could avoid ent_w_req_queue and ent_commit_queue
> if we use fpq->processing, but processing consists of 256 lists -
> overhead is much smaller by using the entry lists?
>
>
> >
> >>
> >> And we also need to iterate over
> >>
> >> fuse_req_queue
> >> fuse_req_bg_queue
> >
> > Why do we need to iterate through fuse_req_queue and
> > fuse_req_bg_queue? fuse_uring_request_expired() checks the head of
> > fuse_req_queue and fuse_req_bg_queue and given that requests are added
> > to fuse_req_queue/fuse_req_bg_queue sequentially (eg added to the tail
> > of these lists), why isn't this enough?
>
> I admit I'm a bit lost with that question. Aren't you pointing out
> the same lists as I do?
>

Oh, I thought your comment was saying that we need to "iterate" over
it (eg go through every request on the lists)? It currently already
checks the fuse_req_queue and fuse_req_bg_queue lists (in
fuse_uring_request_expired() which gets invoked in the
fuse_check_timeout() timeout handler).

Maybe the fuse_uring_request_expired() addition got missed - I tried
to call this out in the cover letter changelog, as I had to rebase
this patchset on top of the io-uring patches, so I added this function
in to make it work for io-uring. I believe this suffices for now for
the io uring case (with future optimizations that can be added)?

Thanks,
Joanne

> >
> >
> > If it's helpful, I can resubmit this patch series so that the io-uring
> > changes are isolated to its own patch (eg have patch 1 and 2 from the
> > original series and then have patch 3 be the io-uring changes).
>
> Sounds good to me.
>
>
> Thanks,
> Bernd