Re: [PATCH] net/mlx5: fix FD configuration for Rx interrupt

[Thomas Monjalon <thomas@monjalon.net>]

10/03/2022 16:12, David Marchand:
> On Thu, Mar 10, 2022 at 2:19 PM Michael Baum <michaelba@nvidia.com> wrote:
> >
> > The mlx5_rx_intr_vec_enable() function allocates queue vector and fill
> > FD list for Rx interrupts.
> >
> > The driver wrongly configured the FD with a non-blocking flag which
> > prevent waiting on this FD.
> >
> > This patch removes O_NONBLOCK flag adding.
> 
> - Maybe I deserve a Reported-by: credit on this issue.
> I sent a proposal to make use of Rx interrupts in OVS
> https://patchwork.ozlabs.org/project/openvswitch/patch/20220304161132.22065-1-david.marchand@redhat.com/.
> And that's when I noticed that mlx5 rx fds were waking OVS too often
> and reported it to mlx5 maintainers.
> 
> 
> - Testing this patch by starting dpdk-l3fwd-power example (and no
> traffic sent at all):
> 
> # strace -r -f ./dpdk-dir/v21.11/examples/dpdk-l3fwd-power --lcores
> 0@3,1@5 -a 0000:82:00.0 --in-memory -- -p 0x1 -P --config '(0,0,1)'
> ...
> [pid 534983]      0.000348 epoll_wait(26, [], 1, 10) = 0
> [pid 534983]      0.010082 read(24,
> 
> For some reason, there is an event available for fd 18 right away
> (which is the issue in the first place).
> When reading this fd, read() blocks until an actual packet is received.
> 
> Then, I send exactly one packet:
> [pid 534983]      0.010082 read(24, "@\217:\370\21\0\0\0", 136) = 8
> [pid 534983]      9.228478 epoll_wait(26, [], 1, 10) = 0
> [pid 534983]      0.010082 read(24,
> 
> That makes mlx5 rx interrupts unusable for an application that does
> more than just polling one rxq.

Excuse me, I don't understand this trace.
What is the first read?
Having a second read after epoll_wait is normal if a packet is received,
isn't it?