netdev - Re: [PATCH net-next] net: netdev_alloc_skb() use build

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Message-ID: <1338822064.2760.1834.camel@edumazet-glaptop>
Date:	Mon, 04 Jun 2012 17:01:04 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Willy Tarreau <w@....eu>, David Miller <davem@...emloft.net>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: netdev_alloc_skb() use build_skb()

On Mon, 2012-06-04 at 17:17 +0300, Michael S. Tsirkin wrote:

> bnx2 and tg3 both do skb_reserve of at least NET_SKB_PAD
> after build_skb. You are saying it's not a must?
> 

32 would be the minimum. NETS_SKB_PAD is using a cache line (64 bytes
on most x86 current cpus) to avoid using half a cache line.

> Hmm so maybe we should teach the hypervisor to write data
> out at an offset. Interesting.
> 
> Another question is about very small packets truesize.
> build_skb sets truesize to frag_size but isn't
> this too small? We keep the whole page around, no?

We keep one page per cpu, at most.

For example on MTU=1500 and PAGE_SIZE=4096, one page is splitted into
two fragments, of 1500 + NET_SKB_PAD + align(shared_info), so its good
enough (this is very close from 2048 'truesize')

But yes, for some uses (wifi for example), we might use a full page per
skb, yet underestimate skb->truesize. Hopefully we can track these uses
and fix them.

ath9k for example could be changed, to be able to reassemble up to 3
frags instead of 2 frags, ie extending what did commit
0d95521ea74735826cb2e28bebf6a07392c75bfa (ath9k: use split rx
buffers to get rid of order-1 skb allocations)



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