netdev - RE: [PATCH v1 6/6] net: fec: Add software TSO support

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Date:	Sat, 31 May 2014 18:39:30 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"fugang.duan@...escale.com" <fugang.duan@...escale.com>
Cc:	"Frank.Li@...escale.com" <Frank.Li@...escale.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"ezequiel.garcia@...e-electrons.com" 
	<ezequiel.garcia@...e-electrons.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"bhutchings@...arflare.com" <bhutchings@...arflare.com>,
	"stephen@...workplumber.org" <stephen@...workplumber.org>
Subject: RE: [PATCH v1 6/6] net: fec: Add software TSO support

On Sun, 2014-06-01 at 00:55 +0000, fugang.duan@...escale.com wrote:
> From: Eric Dumazet <eric.dumazet@...il.com> Data: Saturday, May 31, 2014 12:22 AM
> >
> >How cpu handles misaligned 32bit accesses ?
> 
> You mean use extra descriptor for misaligned bytes ? Or attach the misaligned bytes to header descriptor ?

I meant, maybe you could not force alignment of the IP/TCP headers on a
4byte boundary, but using +2 bytes offset

This way, frames would be 4-bytes aligned from the Etnernet Header.

> So I connect to APPLE MAC book to test again, test result (applied the patches to our internal kernel 3.10.31):
> High mem disable: tx bandwidth 942Mbps, cpu loading is 65%.
> High mem enable: tx bandwidth 930Mbps, cpu loading is 100%. 
> => I don't know why kernel highmem config enable cause so much performance drop ???
> 
> For your above suggestion "using one descriptor per MSS, instead of two":
> Yes, for imx6dl soc, we just do it like this.  For imx6sx soc FEC that support byte alignment,  so it also use one descriptor per MSS.
> 
> Thanks for your suggestion and response. Do you know why highmem cause much performance drop for SW TSO ?

Check NETIF_F_HIGHDMA : your driver might be able to advertise its
support.

Check your copies, because you might then need kmap()

For an example, read efx_skb_copy_bits_to_pio() in
drivers/net/ethernet/sfc/tx.c



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