On Wed, 11 Oct 2006 21:11:38 +0100
Steven Whitehouse <[EMAIL PROTECTED]> wrote:
> Hi,
>
> On Wed, Oct 11, 2006 at 05:01:03PM +0200, Michael S. Tsirkin wrote:
> > Quoting Steven Whitehouse <[EMAIL PROTECTED]>:
> > > > ssize_t tcp_sendpage(struct socket *sock, struct page *page, int offset,
> > > > size_t size, int flags)
> > > > {
> > > > ssize_t res;
> > > > struct sock *sk = sock->sk;
> > > >
> > > > if (!(sk->sk_route_caps & NETIF_F_SG) ||
> > > > !(sk->sk_route_caps & NETIF_F_ALL_CSUM))
> > > > return sock_no_sendpage(sock, page, offset, size,
> > > > flags);
> > > >
> > > >
> > > > So, it seems that if I set NETIF_F_SG but clear NETIF_F_ALL_CSUM,
> > > > data will be copied over rather than sent directly.
> > > > So why does dev.c have to force set NETIF_F_SG to off then?
> > > >
> > > I agree with that analysis,
> >
> > So, would you Ack something like the following then?
> >
>
> In so far as I'm able to ack it, then yes, but with the following
> caveats: that you also need to look at the tcp code's checks for
> NETIF_F_SG (aside from the interface to tcp_sendpage which I think
> we've agreed is ok) and ensure that this patch will not change their
> behaviour, and here I'm thinking of the test in net/ipv4/tcp.c:select_size()
> in particular - there may be others but thats the only one I can think
> of off the top of my head. I think this is what davem was getting at
> with his comment about copy & sum for smaller packets.
>
> Also all subject to approval by davem and shemminger of course :-)
>
> My general feeling is that devices should advertise the features that
> they actually have and that the protocols should make the decision
> as to which ones to use or not depending on the combinations available
> (which I think is pretty much your argument).
>
> Steve.
>
You might want to try ignoring the check in dev.c and testing
to see if there is a performance gain. It wouldn't be hard to test
a modified version and validate the performance change.
You could even do what I suggested and use skb_checksum_help()
to do inplace checksumming, as a performance test.
--
Stephen Hemminger <[EMAIL PROTECTED]>
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