linux-kernel - Re: [PATCH] xen/netback: calculate correctly the SKB slots.

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Message-ID: <20120522180121.GB22488@phenom.dumpdata.com>
Date:	Tue, 22 May 2012 14:01:21 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	xen-devel@...ts.xensource.com, ian.campbell@...rix.com,
	netdev@...r.kernel.org, davem@...emloft.net,
	linux-kernel@...r.kernel.org,
	Adnan Misherfi <adnan.misherfi@...cle.com>
Subject: Re: [PATCH] xen/netback: calculate correctly the SKB slots.

On Mon, May 21, 2012 at 08:14:00PM +0100, Ben Hutchings wrote:
> On Mon, 2012-05-21 at 13:36 -0400, Konrad Rzeszutek Wilk wrote:
> > From: Adnan Misherfi <adnan.misherfi@...cle.com>
> > 
> > A programming error cause the calculation of receive SKB slots to be
> > wrong, which caused the RX ring to be erroneously declared full,
> > and the receive queue to be stopped. The problem shows up when two
> > guest running on the same server tries to communicates using large
> > MTUs. Each guest is connected to a bridge with VLAN over bond
> > interface, so traffic from one guest leaves the server on one bridge
> > and comes back to the second guest on the second bridge. This can be
> > reproduces using ping, and one guest as follow:
> > 
> > - Create active-back bond (bond0)
> > - Set up VLAN 5 on bond0 (bond0.5)
> > - Create a bridge (br1)
> > - Add bond0.5 to a bridge (br1)
> > - Start a guest and connect it to br1
> > - Set MTU of 9000 across the link
> > 
> > Ping the guest from an external host using packet sizes of 3991, and
> > 4054; ping -s 3991 -c 128 "Guest-IP-Address"
> > 
> > At the beginning ping works fine, but after a while ping packets do
> > not reach the guest because the RX ring becomes full, and the queue
> > get stopped. Once the problem accrued, the only way to get out of it
> > is to reboot the guest, or use xm network-detach/network-attach.
> > 
> > ping works for packets sizes 3990,3992, and many other sizes including
> > 4000,5000,9000, and 1500 ..etc. MTU size of 3991,4054 are the sizes
> > that quickly reproduce this problem.
> > 
> > Signed-off-by: Adnan Misherfi <adnan.misherfi@...cle.com>
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > ---
> >  drivers/net/xen-netback/netback.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> > index 957cf9d..e382e5b 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -212,7 +212,7 @@ unsigned int xenvif_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
> 
> The function name is xen_netbk_count_skb_slots() in net-next.  This
> appears to depend on the series in
> <http://lists.xen.org/archives/html/xen-devel/2012-01/msg00982.html>.

Ah, this was based off 3.4.

> 
> >  	int i, copy_off;
> >  
> >  	count = DIV_ROUND_UP(
> > -			offset_in_page(skb->data)+skb_headlen(skb), PAGE_SIZE);
> > +			offset_in_page(skb->data + skb_headlen(skb)), PAGE_SIZE);
> 
> The new version would be equivalent to:
> 	count = offset_in_page(skb->data + skb_headlen(skb)) != 0;
> which is not right, as netbk_gop_skb() will use one slot per page.
> 
> The real problem is likely that you're not using the same condition to
> stop and wake the queue.  Though it appears you're also missing an

Hmm..
> smp_mb() at the top of xenvif_notify_tx_completion().
> 
> Ben.
> 
> >  	copy_off = skb_headlen(skb) % PAGE_SIZE;
> >  
> 
> -- 
> Ben Hutchings, Staff Engineer, Solarflare
> Not speaking for my employer; that's the marketing department's job.
> They asked us to note that Solarflare product names are trademarked.
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