netdev - [RFC] truesize lies

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Date:	Sun, 09 Oct 2011 21:02:28 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	netdev <netdev@...r.kernel.org>
Subject: [RFC] truesize lies

I noticed on my laptop a difference if I use wifi or wired internet
connectivity.

On wifi mode, I can see netstat -s giving sign of bad rcvbuf tuning :

   371 packets collapsed in receive queue due to low socket buffer

After some analysis, I found full sized tcp skbs (len=1440) had a 2864
bytes truesize on my wifi adapter. Ouch...

On tg3 adapter, truesize is MTU + NET_SKB_PAD + sizeof(sk_buff).

On some devices (say... NIU, but other drivers have same problem :
bnx2x, ), truesize is sizeof(sk_buff) + frame_length.

So if 'truesize' really means to account exact memory cost of frames
(including the empty space after used part), I would say :

1) NIU is lying (it should account in niu_rx_skb_append() not the used
part of the page, but reserved part : PAGE_SIZE/rbr_blocks_per_page)

2) Autotuning is a bit pessimistic : It works only if truesize is MSS +
sizeof(sk_buff) + 16 + MAX_TCP_HEADER

I guess most driver writers adjust truesize to please TCP stack and get
nice performance numbers.

skb->truesize = frame_len + sizeof(sk_buff);

What should we do exactly ?

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