Re: [PATCH bpf-next v4 3/3] selftests/bpf: Support nonblock for send_recv_data

[Martin KaFai Lau <martin.lau@linux.dev>]

On 4/9/24 11:13 PM, Geliang Tang wrote:
> From: Geliang Tang <tanggeliang@kylinos.cn>
> 
> Some tests, such as the MPTCP bpf tests, require send_recv_data helper
> to run in nonblock mode.
> 
> This patch adds nonblock support for send_recv_data(). Check if it is
> currently in nonblock mode, and if so, ignore EWOULDBLOCK to continue
> sending and receiving.
> 
> Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
> ---
>   tools/testing/selftests/bpf/network_helpers.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/network_helpers.c b/tools/testing/selftests/bpf/network_helpers.c
> index 137cd18ef3f2..ca16ef2b648e 100644
> --- a/tools/testing/selftests/bpf/network_helpers.c
> +++ b/tools/testing/selftests/bpf/network_helpers.c
> @@ -555,6 +555,7 @@ struct send_recv_arg {
>   static void *send_recv_server(void *arg)
>   {
>   	struct send_recv_arg *a = (struct send_recv_arg *)arg;
> +	int flags = fcntl(a->fd, F_GETFL);
>   	ssize_t nr_sent = 0, bytes = 0;
>   	char batch[1500];
>   	int err = 0, fd;
> @@ -578,6 +579,8 @@ static void *send_recv_server(void *arg)
>   		if (nr_sent == -1 && errno == EINTR)
>   			continue;
>   		if (nr_sent == -1) {
> +			if (flags & O_NONBLOCK && errno == EWOULDBLOCK)

I still don't see why it needs to be a non blocking IO. mptcp should work
with blocking IO also, no? Does it really need non blocking IO to make
mptcp test work? I would rather stay with blocking IO in selftest as much as
possible for simplicity reason.

I am afraid the root cause of the EAGAIN thread has not been figured out yet:
https://lore.kernel.org/all/b3943f9a8bf595212b00e96ba850bf32893312cc.camel@kernel.org/

Lets drop patch 3 until it is understood why mptcp needs EAGAIN or non-blocking IO.
It feels like there is some flakiness and it should be understood and avoided.

Other than the comment in patch 2, the first two patches lgtm. Please respin with
the first two patches.

> +				continue;
>   			err = -errno;
>   			break;
>   		}
> @@ -599,6 +602,7 @@ static void *send_recv_server(void *arg)
>   
>   int send_recv_data(int lfd, int fd, uint32_t total_bytes)
>   {
> +	int flags = fcntl(lfd, F_GETFL);
>   	ssize_t nr_recv = 0, bytes = 0;
>   	struct send_recv_arg arg = {
>   		.fd	= lfd,
> @@ -622,8 +626,11 @@ int send_recv_data(int lfd, int fd, uint32_t total_bytes)
>   			       MIN(total_bytes - bytes, sizeof(batch)), 0);
>   		if (nr_recv == -1 && errno == EINTR)
>   			continue;
> -		if (nr_recv == -1)
> +		if (nr_recv == -1) {
> +			if (flags & O_NONBLOCK && errno == EWOULDBLOCK)
> +				continue;
>   			break;
> +		}
>   		bytes += nr_recv;
>   	}
>