On Fri, Jan 09, 2009 at 11:12:09PM +0100, Eric Dumazet wrote:

quoted text
> Willy Tarreau a écrit :

> > On Fri, Jan 09, 2009 at 10:24:00PM +0100, Willy Tarreau wrote:

> >> On Fri, Jan 09, 2009 at 09:51:17PM +0100, Eric Dumazet wrote:

> >> (...)

> >>>> Also, in your second mail, you're saying that your change

> >>>> might return more data than requested by the user. I can't

> >>>> find why, could you please explain to me, as I'm still quite

> >>>> ignorant in this area ?

> >>> Well, I just tested various user programs and indeed got this

> >>> strange result :

> >>>

> >>> Here I call splice() with len=1000 (0x3e8), and you can see

> >>> it gives a result of 1460 at the second call.

> >

> > OK finally I could reproduce it and found why we have this. It's

> > expected in fact.

> >

> > The problem when we loop in tcp_read_sock() is that tss->len is

> > not decremented by the amount of bytes read, this one is done

> > only in tcp_splice_read() which is outer.

> >

> > The solution I found was to do just like other callers, which means

> > use desc->count to keep the remaining number of bytes we want to

> > read. In fact, tcp_read_sock() is designed to use that one as a stop

> > condition, which explains why you first had to hide it.

> >

> > Now with the attached patch as a replacement for my previous one,

> > both issues are solved :

> >   - I splice 1000 bytes if I ask to do so

> >   - I splice as much as possible if available (typically 23 kB).

> >

> > My observed performances are still at the top of earlier results

> > and IMHO that way of counting bytes makes sense for an actor called

> > from tcp_read_sock().

> >

> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c

> > index 35bcddf..51ff3aa 100644

> > --- a/net/ipv4/tcp.c

> > +++ b/net/ipv4/tcp.c

> > @@ -522,8 +522,12 @@ static int tcp_splice_data_recv(read_descriptor_t *rd_desc, struct sk_buff *skb,

> >  				unsigned int offset, size_t len)

> >  {

> >  	struct tcp_splice_state *tss = rd_desc->arg.data;

> > +	int ret;

> >

> > -	return skb_splice_bits(skb, offset, tss->pipe, tss->len, tss->flags);

> > +	ret = skb_splice_bits(skb, offset, tss->pipe, rd_desc->count, tss->flags);

> > +	if (ret > 0)

> > +		rd_desc->count -= ret;

> > +	return ret;

> >  }

> >

> >  static int __tcp_splice_read(struct sock *sk, struct tcp_splice_state *tss)

> > @@ -531,6 +535,7 @@ static int __tcp_splice_read(struct sock *sk, struct tcp_splice_state *tss)

> >  	/* Store TCP splice context information in read_descriptor_t. */

> >  	read_descriptor_t rd_desc = {

> >  		.arg.data = tss,

> > +		.count = tss->len,

> >  	};

> >

> >  	return tcp_read_sock(sk, &rd_desc, tcp_splice_data_recv);

> >

>

> OK, I came to a different patch. Please check other tcp_read_sock() callers in tree :)

I've seen the other callers, but they all use desc->count for their own

purpose. That's how I understood what it was used for :-)
I think it's better not to change the API here and use tcp_read_sock()

how it's supposed to be used. Also, the less parameters to the function,

the better.
However I'm OK for the !timeo before release_sock/lock_sock. I just

don't know if we can put the rest of the if above or not. I don't

know what changes we're supposed to collect by doing release_sock/

lock_sock before the if().
Regards,

Willy
--

To unsubscribe from this list: send the line "unsubscribe netdev" in

the body of a message to majordomo@vger.kernel.org

More majordomo info at  http://vger.kernel.org/majordomo-info.html