On 9/17/22 11:24, Stefan Metzmacher wrote:
Am 17.09.22 um 11:22 schrieb Pavel Begunkov:
On 9/16/22 22:36, Stefan Metzmacher wrote:
The 2nd cqe for IORING_OP_SEND_ZC has IORING_CQE_F_NOTIF set in cqe->flags
and it will now have the number of successful completed
io_uring_tx_zerocopy_callback() callbacks in the lower 31-bits
of cqe->res, the high bit (0x80000000) is set when
io_uring_tx_zerocopy_callback() was called with success=false.
It has a couple of problems, and because that "simplify uapi"
patch is transitional it doesn't go well with what I'm queuing
for 6.1, let's hold it for a while.
Once the current behavior gets released stable, we're no
longer able to change the meaning of cqe.res.
As cqe.res == 0 would mean zero copy wasn't used at all,
which would be the indication for userspace to avoid using SEND_ZC.
But if 6.0 would always return cqe.res = 0, there's no chance for
userspace to have a detection strategy.
And I don't think it will cause a lot of trouble for your 6.1 stuff (assuming
you mean your SENDMSG_ZC code), I was already having that on top
of my test branches, the current one is:
https://git.samba.org/?p=metze/linux/wip.git;a=shortlog;h=refs/heads/io_uring-6.0.0-rc5-metze.08
Not that one though, 1) I want to shrink ubuf_info as we're a bit out
of space on the io_uring side and it prevents some embedding, so there
won't be additional fields you used. And 2) we want to have a feature
merging completion + notif CQEs into one (useful for UDP and some TCP
cases), but that would mean we'll be using cqe->res for the normal
return value.
We can disable the success counting in this case, but it's not nice,
and we can't actually count in io_uring_tx_zerocopy_callback() as in
the patch (racy). Also, the callback will be called multiple times for
a number of different reasons like io_uring flush or net splitting skbs.
The number won't be much useful and unnecessary exposes internal details,
so I think F_COPIED in cqe->flags is a better option.
It's a good question though whether we need more versatile reporting and
how to do it right. Probably should be optional and not a part of IO path,
e.g. send(MSG_PROBE) / ioctl / proc stats / etc.
I plan to test SENDMSG_ZC with Samba next week.
Awesome
--
Pavel Begunkov
