Re: [External] [PATCH v13 3/8] QIOChannelSocket: Implement io_writev zero copy flag & io_flush for CONFIG_LINUX

[Leonardo Bras Soares Passos]

Hello Peter,

On Wed, Jun 1, 2022 at 10:58 AM Peter Xu <peterx@redhat.com> wrote:
>
[...]
> > Hi, Leonardo. I'm also paying attention to the application of MSG_ZEROCOPY
> > in live migration recently. I noticed that you defined a member
> > `zero_copy_queued` in the struct QIOChannelSocket, but I can't find out
> > where the value of this member has been changed in your patch. Can you
> > answer it for me?
> >
>
> Good point.. it should probably be increased when queuing the pages. We'd
> better fix it up or it seems the flush() will be literally an no-op..

That's correct.
I am working on a fix right now.
The idea is to increment it in qio_channel_socket_writev() if sendmsg succeeds.

>
> Two things in qio_channel_socket_flush() we can do to make sure it'll work
> as expected, imo:
>
>   1) make ret=-1 as initial value, rather than 1 - we only check negative
>      errors in the caller so we could have missed a positive "1"
>
>   2) add a tracepoint into the loop of updating zero_copy_sent
>
> Leo, what's your take?

(1) is not an option, as the interface currently uses ret=1 to make
sure MSG_ZEROCOPY is getting used,
I added that so the user of qio_channel can switch off zero-copy if
it's not getting used, and save some cpu.

(2) is not a problem, but I fail to see how useful that would be. Is
the idea manually keeping track of flush happening?

Best regards,
Leo