On Tue, Apr 12, 2022 at 7:27 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > > On 4/12/22 8:19 PM, Eric Dumazet wrote: > > On Tue, Apr 12, 2022 at 7:12 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > >> > >> On 4/12/22 8:05 PM, Eric Dumazet wrote: > >>> On Tue, Apr 12, 2022 at 7:01 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > >>>> > >>>> On 4/12/22 7:54 PM, Eric Dumazet wrote: > >>>>> On Tue, Apr 12, 2022 at 6:26 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > >>>>>> > >>>>>> On 4/12/22 6:40 PM, Eric Dumazet wrote: > >>>>>>> > >>>>>>> On 4/12/22 13:26, Jens Axboe wrote: > >>>>>>>> Hi, > >>>>>>>> > >>>>>>>> If we accept a connection directly, eg without installing a file > >>>>>>>> descriptor for it, or if we use IORING_OP_SOCKET in direct mode, then > >>>>>>>> we have a socket for recv/send that we can fully serialize access to. > >>>>>>>> > >>>>>>>> With that in mind, we can feasibly skip locking on the socket for TCP > >>>>>>>> in that case. Some of the testing I've done has shown as much as 15% > >>>>>>>> of overhead in the lock_sock/release_sock part, with this change then > >>>>>>>> we see none. > >>>>>>>> > >>>>>>>> Comments welcome! > >>>>>>>> > >>>>>>> How BH handlers (including TCP timers) and io_uring are going to run > >>>>>>> safely ? Even if a tcp socket had one user, (private fd opened by a > >>>>>>> non multi-threaded program), we would still to use the spinlock. > >>>>>> > >>>>>> But we don't even hold the spinlock over lock_sock() and release_sock(), > >>>>>> just the mutex. And we do check for running eg the backlog on release, > >>>>>> which I believe is done safely and similarly in other places too. > >>>>> > >>>>> So lets say TCP stack receives a packet in BH handler... it proceeds > >>>>> using many tcp sock fields. > >>>>> > >>>>> Then io_uring wants to read/write stuff from another cpu, while BH > >>>>> handler(s) is(are) not done yet, > >>>>> and will happily read/change many of the same fields > >>>> > >>>> But how is that currently protected? > >>> > >>> It is protected by current code. > >>> > >>> What you wrote would break TCP stack quite badly. > >> > >> No offense, but your explanations are severely lacking. By "current > >> code"? So what you're saying is that it's protected by how the code > >> currently works? From how that it currently is? Yeah, that surely > >> explains it. > >> > >>> I suggest you setup/run a syzbot server/farm, then you will have a > >>> hundred reports quite easily. > >> > >> Nowhere am I claiming this is currently perfect, and it should have had > >> an RFC on it. Was hoping for some constructive criticism on how to move > >> this forward, as high frequency TCP currently _sucks_ in the stack. > >> Instead I get useless replies, not very encouraging. > >> > >> I've run this quite extensively on just basic send/receive over sockets, > >> so it's not like it hasn't been run at all. And it's been fine so far, > >> no ill effects observed. If we need to tighten down the locking, perhaps > >> a valid use would be to simply skip the mutex and retain the bh lock for > >> setting owner. As far as I can tell, should still be safe to skip on > >> release, except if we need to process the backlog. And it'd serialize > >> the owner setting with the BH, which seems to be your main objection in. > >> Mostly guessing here, based on the in-depth replies. > >> > >> But it'd be nice if we could have a more constructive dialogue about > >> this, rather than the weird dismisiveness. > >> > >> > > > > Sure. It would be nice that I have not received such a patch series > > the day I am sick. > > I'm sorry that you are sick - but if you are not in a state to reply, > then please just don't. It sets a bad example. It was sent to the list, > not to you personally. I tried to be as constructive as possible, and Jakub pinged me about this series, so I really thought Jakub was okay with it. So I am a bit concerned. > > Don't check email then, putting the blame on ME for posting a patchset > while you are sick is uncalled for and rude. If I had a crystal ball, I > would not be spending my time working on the kernel. You know what > would've been a better idea? Replying that you are sick and that you are > sorry for being an ass on the mailing list. Wow. > > > Jakub, David, Paolo, please provide details to Jens, thanks. > > There's no rush here fwiw - I'm heading out on PTO rest of the week, > so we can pick this back up when I get back. I'll check in on emails, > but activity will be sparse. > > -- > Jens Axboe >
