Re: [External] Re: [PATCH v3 2/3] io_uring: avoid ring quiesce while registering/unregistering eventfd

[Pavel Begunkov]

On 2/3/22 22:16, Jens Axboe wrote:
> On 2/3/22 2:47 PM, Pavel Begunkov wrote:
> > On 2/3/22 19:54, Usama Arif wrote:
> > > On 03/02/2022 19:06, Jens Axboe wrote:
> > > > On 2/3/22 12:00 PM, Pavel Begunkov wrote:
> > > > > On 2/3/22 18:29, Jens Axboe wrote:
> > > > > > On 2/3/22 11:26 AM, Usama Arif wrote:
> > > > > > > Hmm, maybe i didn't understand you and Pavel correctly. Are you
> > > > > > > suggesting to do the below diff over patch 3? I dont think that would be
> > > > > > > correct, as it is possible that just after checking if ctx->io_ev_fd is
> > > > > > > present unregister can be called by another thread and set ctx->io_ev_fd
> > > > > > > to NULL that would cause a NULL pointer exception later? In the current
> > > > > > > patch, the check of whether ev_fd exists happens as the first thing
> > > > > > > after rcu_read_lock and the rcu_read_lock are extremely cheap i believe.
> > > > > >
> > > > > > They are cheap, but they are still noticeable at high requests/sec
> > > > > > rates. So would be best to avoid them.
> > > > > >
> > > > > > And yes it's obviously racy, there's the potential to miss an eventfd
> > > > > > notification if it races with registering an eventfd descriptor. But
> > > > > > that's not really a concern, as if you register with inflight IO
> > > > > > pending, then that always exists just depending on timing. The only
> > > > > > thing I care about here is that it's always _safe_. Hence something ala
> > > > > > what you did below is totally fine, as we're re-evaluating under rcu
> > > > > > protection.
> > > > >
> > > > > Indeed, the patch doesn't have any formal guarantees for propagation
> > > > > to already inflight requests, so this extra unsynchronised check
> > > > > doesn't change anything.
> > > > >
> > > > > I'm still more сurious why we need RCU and extra complexity when
> > > > > apparently there is no use case for that. If it's only about
> > > > > initial initialisation, then as I described there is a much
> > > > > simpler approach.
> > > >
> > > > Would be nice if we could get rid of the quiesce code in general, but I
> > > > haven't done a check to see what'd be missing after this...
> > >
> > > I had checked! I had posted below in in reply to v1 (https://lore.kernel.org/io-uring/02fb0bc3-fc38-b8f0-3067-edd2a525ef29@xxxxxxxxx/T/#m5ac7867ac61d86fe62c099be793ffe5a9a334976), but i think it got missed! Copy-pasting here for reference:
> >
> > May have missed it then, apologies
> >
> > > "
> > > I see that if we remove ring quiesce from the the above 3 opcodes, then
> > > only IORING_REGISTER_ENABLE_RINGS and IORING_REGISTER_RESTRICTIONS is
> > > left for ring quiesce. I just had a quick look at those, and from what i
> > > see we might not need to enter ring quiesce in
> > > IORING_REGISTER_ENABLE_RINGS as the ring is already disabled at that point?
> > > And for IORING_REGISTER_RESTRICTIONS if we do a similar approach to
> > > IORING_REGISTER_EVENTFD, i.e. wrap ctx->restrictions inside an RCU
> > > protected data structure, use spin_lock to prevent multiple
> > > io_register_restrictions calls at the same time, and use read_rcu_lock
> > > in io_check_restriction, then we can remove ring quiesce from
> > > io_uring_register altogether?
> > >
> > > My usecase only uses IORING_REGISTER_EVENTFD, but i think entering ring
> > > quiesce costs similar in other opcodes. If the above sounds reasonable,
> > > please let me know and i can send patches for removing ring quiesce for
> > > io_uring_register.
> > > "
> > >
> > > Let me know if above makes sense, i can add patches on top of the current patchset, or we can do it after they get merged.
> > >
> > > As for why, quiesce state is very expensive. its making io_uring_register the most expensive syscall in my usecase (~15ms) compared to ~0.1ms now with RCU, which is why i started investigating this. And this patchset avoids ring quiesce for 3 of the opcodes, so it would generally be quite helpful if someone does registers and unregisters eventfd multiple times.
> >
> > I agree that 15ms for initial setup is silly and it has to be
> > reduced. However, I'm trying weight the extra complexity against
> > potential benefits of _also_ optimising [de,re]-registration
> >
> > Considering that you only register it one time at the beginning,
> > we risk adding a yet another feature that nobody is going to ever
> > use. This doesn't give me a nice feeling, well, unless you do
> > have a use case.
>
> It's not really a new feature, it's just making the existing one not
> suck quite as much...

Does it matter when nobody uses it? My point is that does not.


> > To emphasise, I'm comparing 15->0.1 improvement for only initial
> > registration (which is simpler) vs 15->0.1 for both registration
> > and unregistration.
>
> reg+unreg should be way faster too, if done properly with the assignment
> tricks.
>
> > fwiw, it alters userpace visible behaviour in either case, shouldn't
> > be as important here but there is always a chance to break userspace
>
> It doesn't alter userspace behavior, if the registration works like I
> described with being able to assign a new one while the old one is being
> torn down.
>
> Or do you mean wrt inflight IO? I don't think the risk is very high
> there, to be honest.

Right, if somebody tries such a trick it'll be pretty confusing to
get randomly firing eventfd, though it's rather a marginal case.

--
Pavel Begunkov