netdev - Re: [RFC PATCH] sock: add SO_RCVQUEUE

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Date:   Mon, 13 Mar 2017 18:34:41 -0500
From:   Josh Hunt <johunt@...mai.com>
To:     David Miller <davem@...emloft.net>
Cc:     edumazet@...gle.com, arnd@...db.de, soheil@...gle.com,
        willemb@...gle.com, pabeni@...hat.com, linux-arch@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [RFC PATCH] sock: add SO_RCVQUEUE_SIZE getsockopt

On 03/13/2017 02:39 PM, David Miller wrote:
> From: Josh Hunt <johunt@...mai.com>
> Date: Mon, 13 Mar 2017 12:38:39 -0500
>
>> On 03/13/2017 11:12 AM, Eric Dumazet wrote:
>>> On Mon, Mar 13, 2017 at 8:59 AM, Josh Hunt <johunt@...mai.com> wrote:
>>>> Allows application to read the amount of data sitting in the receive
>>>> queue.
>>>>
>>>> Signed-off-by: Josh Hunt <johunt@...mai.com>
>>>> ---
>>>>
>>>> A team here is looking for a way to get the amount of data in a UDP
>>>> socket's
>>>> receive queue. It seems like this should be SIOCINQ, but for UDP
>>>> sockets that
>>>> returns the size of the next pending datagram. I implemented the patch
>>>> below,
>>>> but am wondering if this is the right place for this change? I was
>>>> debating
>>>> between this or a new UDP ioctl.
>>>
>>> But what is the 'amount of data' exactly ?
>>> Number of packets, amount of bytes to read from these packets ?
>>
>> I meant bytes. I will clarify in the next version.
>
> As Eric is hinting, the calculation you are using doesn't represent
> this.
>
> You need to do something like walk the receive queue and add the
> skb->len values together.
>
> sk->sk_rmem_alloc is usually much larger than the sum of the skb->len
> values in the socket receive queue.  I don't see how this culmination
> of skb->truesize values is useful, whereas I can see how an application
> could want the summation of the skb->len values.
>

In this particular case they really do want to know total # of bytes in 
the receive queue, not the data bytes they can consume from an 
application pov. The kernel currently only exposes this value through 
netlink or /proc/net/udp from what I saw.

I believe Eric's suggestion in his previous mail was to export all of 
these meminfo metrics via a single socket option call similar to how its 
done in netlink. We could then use that for both call sites.

I agree that it would be useful to also have the data you and Eric are 
suggesting exposed somewhere, the total # of skb->len bytes sitting in 
the receive queue. I could add that as a second socket option.

Does this sound reasonable?

Josh