netdev - Re: [PATCH v1 3/3] usb: gadget: NCM: Stop RX TCP Bursts getting dropped.

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Date:	Thu, 29 May 2014 12:04:23 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jim Baxter <jim_baxter@...tor.com>
Cc:	Felipe Balbi <balbi@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	Bjørn Mork <bjorn@...k.no>,
	David Laight <David.Laight@...LAB.COM>, netdev@...r.kernel.org
Subject: Re: [PATCH v1 3/3] usb: gadget: NCM: Stop RX TCP Bursts getting
 dropped.

On Thu, 2014-05-29 at 18:12 +0100, Jim Baxter wrote:
> This fixes a problem with dropped packets over 16k CDC-NCM
> when the connection is being heavily used.
> 
> The issue was that the skb truesize for the unpacked NCM
> packets was too high after they were cloned from the 16k
> skb, this lead to the potential memory calculated by the
> Kernel running out of memory earlier then it should.
> 
> Signed-off-by: Jim Baxter <jim_baxter@...tor.com>
> ---

Note the patch is OK, but changelog a bit misleading ;)

Kernel was not running out of memory, because truesize was correct.

The problem here is that a frame was consuming more kernel memory than
really needed, so chances of hitting socket sk_rcvbuf limit was high.

BTW :
#define NTB_OUT_SIZE              16384

alloc_skb(size) ->
	kmalloc(16384 + sizeof(struct skb_shared_info)) ->
		roundup() => 32768

So truesize of the skb was infact ~32KB, which is really insane indeed.
After your patch, its back to ~2KB

Acked-by: Eric Dumazet <edumazet@...gle.com>



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