Subject: Re: seeing strange values for tcp sk_rmem_alloc

From: Eric Dumazet <[email-masked]>
Date: Tue, 01 Dec 2009 17:58:17 +0100

Chris Friesen a écrit :
> I'm hoping someone might be able to explain some odd behaviour that I'm
> seeing.
> 
> Some of our developers wanted to be able to see how much of their rx
> socket buffer space was in use, so I added the following to sock_ioctl()
> 
> 
> 		case SIOCGSKRMEMALLOC:
> 		{
> 			int tmp;
> 			err = -EINVAL;
> 			if(!sock->sk)
> 				break;
> 			tmp = atomic_read(&sock->sk->sk_rmem_alloc);
> 			err = copy_to_user(argp, &tmp, sizeof(tmp));
> 			break;
> 		}
> 
> To validate it, I wrote a testcase that opened a tcp socket, then looped
> sending 2k of data at a time to it and calling the above ioctl to check
> the sk_rmem_alloc value (without ever reading from the socket).
> 
> The results were odd--I've copied them below.  Can anyone explain how I
> can send 20K of data but sk_rmem_alloc still only shows 4.8K used, then
> it suddenly jumps by a lot on the next packet to something that more
> reflects reality, then repeats that pattern again?  Is there some
> additional buffering happening somewhere in the TCP stack?
> 

Me wondering why you think sk_rmem_alloc is about TX side.

Its used in RX path. rmem means ReadMemory.

You can send 1 Gbytes of data, and sk_rmem_alloc doesnt change, if your
TCP stream is unidirectionnal.

sk_rmem_alloc grows when skb are queued into receive queue
sk_rmem_alloc shrinks when application reads this receive queue.