netdev - Re: Stable regression with 'tcp: allow splice() to build full TSO packets'

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Date:	Fri, 18 May 2012 00:22:31 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Willy Tarreau <w@....eu>
Cc:	netdev@...r.kernel.org
Subject: Re: Stable regression with 'tcp: allow splice() to build full TSO
 packets'

On Fri, 2012-05-18 at 00:16 +0200, Willy Tarreau wrote:
> On Fri, May 18, 2012 at 12:01:59AM +0200, Eric Dumazet wrote:
> > On Thu, 2012-05-17 at 23:50 +0200, Eric Dumazet wrote:
> > > On Thu, 2012-05-17 at 23:40 +0200, Eric Dumazet wrote:
> > > 
> > > > I dont understand why we should tcp_push() if we sent 0 bytes in this
> > > > splice() call.
> > > > 
> > > > The push() should have be done already by prior splice() call, dont you
> > > > think ?
> > > > 
> > > > out:
> > > > 	if (copied && !(flags & MSG_SENDPAGE_NOTLAST))
> > > > 		tcp_push(sk, flags, mss_now, tp->nonagle);
> > > > 
> > > 
> > > I think I now understand
> > > 
> > > One splice() syscall actually calls do_tcp_sendpages() several times
> > > (N).
> > > 
> > > Problem is N-1 calls are done with MSG_SENDPAGE_NOTLAST set
> > > 
> > > And last one with MSG_SENDPAGE_NOTLAST unset
> > > 
> > > So maybe we should replace this test by :
> > > 
> > > 	if (!(flags & MSG_SENDPAGE_NOTLAST))
> > > 		tcp_push(...);
> > > 
> > > 
> > 
> > Its more tricky than that.
> > 
> > If we return 0 from do_tcp_sendpages(), __splice_from_pipe() wont call
> > us again, but we need to flush().  (This bug is indeed very old)
> > 
> > So maybe use :
> > 
> > if ((copied && !(flags & MSG_SENDPAGE_NOTLAST)) ||
> >     !copied)
> > 	tcp_push(...);
> 
> That's what I wanted to do at first but was still worried about the
> situations where we fail upon first call due to lack of memory. And
> indeed that did not fix the issue either :-(
> 
> At least now I've checked that we fail here in do_tcp_sendpages() :
> 
> 	if (!sk_stream_memory_free(sk)) {
> 		printk(KERN_DEBUG "%s:%d copied=%d\n", __FUNCTION__, __LINE__, copied);
> 		goto wait_for_sndbuf;
> 	}
> 
> Well, finally I just put the same test as yours above for the call to
> tcp_push() upon out of memory and it fixed it too. I have simplified
> the expression, since ((A && !B) || !A) == !(A & B).
> 
> With the updated patch here it works for me. I don't know if it's
> better.
> 
> Willy
> 
> -----
> 
> From 1f26263bd4f8827bbca4cef77a99fac128cf8ccc Mon Sep 17 00:00:00 2001
> From: Willy Tarreau <w@....eu>
> Date: Thu, 17 May 2012 22:43:20 +0200
> Subject: [PATCH] tcp: do_tcp_sendpages() must try to push data out on oom conditions
> 
> Since recent changes on TCP splicing (starting with commits 2f533844
> and 35f9c09f), I started seeing massive stalls when forwarding traffic
> between two sockets using splice() when pipe buffers were larger than
> socket buffers.
> 
> Latest changes (net: netdev_alloc_skb() use build_skb()) made the
> problem even more apparent.
> 
> The reason seems to be that if do_tcp_sendpages() fails on out of memory
> condition without being able to send at least one byte, tcp_push() is not
> called and the buffers cannot be flushed.
> 
> After applying the attached patch, I cannot reproduce the stalls at all
> and the data rate it perfectly stable and steady under any condition
> which previously caused the problem to be permanent.
> 
> The issue seems to have been there since before the kernel migrated to
> git, which makes me think that the stalls I occasionally experienced
> with tux during stress-tests years ago were probably related to the
> same issue.
> 
> This issue was first encountered on 3.0.31 and 3.2.17, so please backport
> to -stable.
> 
> Signed-off-by: Willy Tarreau <w@....eu>
> Cc: <stable@...r.kernel.org>
> ---
>  net/ipv4/tcp.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 63ddaee..cfe47c1 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -917,7 +917,7 @@ new_segment:
>  wait_for_sndbuf:
>  		set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
>  wait_for_memory:
> -		if (copied)
> +		if (!(copied && (flags & MSG_SENDPAGE_NOTLAST)))
>  			tcp_push(sk, flags & ~MSG_MORE, mss_now, TCP_NAGLE_PUSH);
>  
>  		if ((err = sk_stream_wait_memory(sk, &timeo)) != 0)
> @@ -927,7 +927,7 @@ wait_for_memory:
>  	}
>  
>  out:
> -	if (copied && !(flags & MSG_SENDPAGE_NOTLAST))
> +	if (!(copied && (flags & MSG_SENDPAGE_NOTLAST)))
>  		tcp_push(sk, flags, mss_now, tp->nonagle);
>  	return copied;
>  


Hmm... I believe I prefer your prior patch ( the one I Acked)



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