netdev - Re: Major network performance regression in 3.7

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Message-ID: <20130106173543.GB22432@1wt.eu>
Date:	Sun, 6 Jan 2013 18:35:43 +0100
From:	Willy Tarreau <w@....eu>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Major network performance regression in 3.7

On Sun, Jan 06, 2013 at 09:10:55AM -0800, Eric Dumazet wrote:
> On Sun, 2013-01-06 at 17:44 +0100, Willy Tarreau wrote:
> > On Sun, Jan 06, 2013 at 08:39:53AM -0800, Eric Dumazet wrote:
> > > Hmm, I'll have to check if this really can be reverted without hurting
> > > vmsplice() again.
> > 
> > Looking at the code I've been wondering whether we shouldn't transform
> > the condition to perform the push if we can't push more segments, but
> > I don't know what to rely on. It would be something like this :
> > 
> >        if (copied &&
> >           (!(flags & MSG_SENDPAGE_NOTLAST) || cant_push_more))
> >                 tcp_push(sk, flags, mss_now, tp->nonagle);
> 
> Good point !
> 
> Maybe the following fix then ?
> 
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 1ca2536..7ba0717 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -941,8 +941,10 @@ out:
>  	return copied;
>  
>  do_error:
> -	if (copied)
> +	if (copied) {
> +		flags &= ~MSG_SENDPAGE_NOTLAST;
>  		goto out;
> +	}
>  out_err:
>  	return sk_stream_error(sk, flags, err);
>  }

Unfortunately it does not work any better, which means to me
that we don't leave via this code path. I tried other tricks
which failed too. I need to understand this part better before
randomly fiddling with it.

Willy

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