Re: Fwd: Querying current tx_queue usage of a SocketCAN interface

From: Tom Evans <tom_usenet@optusnet.com.au>
To: Paarvai Naai <opensource3141@gmail.com>, linux-can@vger.kernel.org
Subject: Re: Fwd: Querying current tx_queue usage of a SocketCAN interface
Date: Mon, 30 Mar 2015 09:42:57 +1100	[thread overview]
Message-ID: <55187FF1.7020701@optusnet.com.au> (raw)
In-Reply-To: <CAE+ymrt-rsXgteuDCvp27BSsQRE5APGDsgsSPhg31Qz3_qZEdQ@mail.gmail.com>

On 29/03/15 07:26, Paarvai Naai wrote:
> Hi,
>
> I have been looking into how to query the SocketCAN interface for its
> current tx_queue usage.

Just letting you know of a (what was for me) unexpected behaviour of the queue.

Sockets can block or return ENOBUFS. Ethernet blocks before it returns ENOBUFS 
like you'd expect. With CAN it does the opposite "out of the box" and needs to 
be fixed.

http://socket-can.996257.n3.nabble.com/Solving-ENOBUFS-returned-by-write-td2886.html

     With Ethernet, the transmit queue length is 1000 (which would
     return ENOBUF) but before that happens it hits SO_SNDBUF,
     which may be 108544, which is the total Data plus SKB, and
     with an SKB size of about 200 that means it blocks at about
     500 before it ENOBUFs at 1000.

     With CAN, it would block at 500, but it ENOBUFs at 10 first with
     the default queue depth!

I do the following to get a 256-deep queue that blocks before it overflows:

     /bin/echo 256 > /sys/class/net/can0/tx_queue_len
     /bin/echo 256 > /sys/class/net/can1/tx_queue_len

     int sndbuf = (250 + 8) * 256;
     socklen_t socklen = sizeof(sndbuf);
     /* Minimum socket buffer to try and get it blocking */
     rc = setsockopt(pSkt->skt, SOL_SOCKET, SO_SNDBUF,
                     &sndbuf, sizeof(sndbuf));