Hi Matt,

On Sat, 2026-08-15 at 00:28 +0200, Matthieu Baerts (NGI0) wrote:
> In case of poll timeout, it might be too late to print the stats
> after
> the socket closure.
> 
> Now, in case of poll timeout, 'ss' and 'nstat' are invoked from
> mptcp_connect to print the stats before exiting. This should help
> debugging poll timeout issues.
> 
> Note: for this "workaround", system() is used for debugging purposes

This print_err_stats() implementation is basically the same as
mptcp_lib_pr_err_stats() in the shell, right?

I think it's better to use mptcp_lib_pr_err_stats() in the shell to
print the information. In mptcp_connect.c, when a poll timeout occurs,
we send a signal to the shell and don't close the socket immediately -
we wait for the shell to finish printing the information before closing
it. What do you think of this approach? There's a reference
implementation in the attachment, but I haven't tested it.

Thanks,
-Geliang

> only. The returned result can then be safely ignored, hence the
> (void)!
> to avoid unused-result warnings.
> 
> Also, 'nstat *Tcp*' will print '#kernel' at the top, but that's OK,
> and
> it seems better to avoid using a pipe here.
> 
> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> ---
> Changes in v3:
> - use double quotes instead of single ones (Sashiko)
> - Link to v2:
> https://patch.msgid.link/20260814-sft-mptcp-stats-b4-close-v2-1-b7a8ab3ddae8@kernel.org
> 
> Changes in v2:
> - use 'ip netns identify' to avoid a warning from AI reviews
> - use 'nstat -s *Tcp*' instead of using grep
> - add a note about (void)!
> - Link to v1:
> https://patch.msgid.link/20260814-sft-mptcp-stats-b4-close-v1-1-2945a591be7e@kernel.org
> ---
>  tools/testing/selftests/net/mptcp/mptcp_connect.c | 15
> +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/tools/testing/selftests/net/mptcp/mptcp_connect.c
> b/tools/testing/selftests/net/mptcp/mptcp_connect.c
> index ea4cb6c1bd5e..d6c2237bdba0 100644
> --- a/tools/testing/selftests/net/mptcp/mptcp_connect.c
> +++ b/tools/testing/selftests/net/mptcp/mptcp_connect.c
> @@ -210,6 +210,19 @@ static void xgetaddrinfo(const char *node, const
> char *service,
>  	}
>  }
>  
> +static void print_err_stats(void)
> +{
> +	char cmd[128];
> +
> +	snprintf(cmd, sizeof(cmd), "ss -Menitam -o '%cport = :%s'
> >&2",
> +		 listen_mode ? 's' : 'd', cfg_port);
> +
> +	fprintf(stderr, "socket stats before socket closure:\n");
> +	(void)!system(cmd);
> +	(void)!system("NSTAT_HISTORY=\"/tmp/$(ip netns
> identify).nstat\" "
> +		      "nstat -s '*Tcp*' >&2");
> +}
> +
>  static void set_rcvbuf(int fd, unsigned int size)
>  {
>  	int err;
> @@ -656,6 +669,7 @@ static int copyfd_io_poll(int infd, int peerfd,
> int outfd,
>  			fprintf(stderr, "%s: poll timed out (events:
> "
>  				"POLLIN %u, POLLOUT %u)\n",
> __func__,
>  				fds.events & POLLIN, fds.events &
> POLLOUT);
> +			print_err_stats();
>  			return 2;
>  		}
>  
> @@ -1194,6 +1208,7 @@ int main_loop_s(int listensock)
>  		return 1;
>  	case 0:
>  		fprintf(stderr, "%s: timed out\n", __func__);
> +		print_err_stats();
>  		close(listensock);
>  		return 2;
>  	}
> 
> ---
> base-commit: d536472a82d18356d323227213acb5603bb9050c
> change-id: 20260814-sft-mptcp-stats-b4-close-20964ff6a6ba
> 
> Best regards,
> --  
> Matthieu Baerts (NGI0) <matttbe@kernel.org>
>