RE: [Xen-devel] network hang trigger

["James Harper" <JamesH@xxxxxxxxxxxxxxxx>]

This lends weight to my idea that it might be a timing related deadlock,
I put some debug statements in to see just how often the drop: path was
called in network_start_xmit (never), and now I almost can't get it to
fail. I put printk's in at the start of the function, and before both
the returns.

This particular machine is not SMP btw. It is a single CPU PII running
at 350Mhz (697 bogomips)

James

> -----Original Message-----
> From: xen-devel-admin@xxxxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
> admin@xxxxxxxxxxxxxxxxxxxxx] On Behalf Of James Harper
> Sent: Thursday, 16 September 2004 10:48
> To: Bin Ren; xen-devel@xxxxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-devel] network hang trigger
> 
> This patch makes no difference on my system. Looking at the line
numbers
> in your patch, your netfront.c appears to be a different vintage to
> mine. I applied the patch manually and then rebuilt the xenU kernel
and
> then booted a domain with it. I haven't touched xen0. Is this the
> correct thing to be doing?
> 
> Ping >mtu from xenU to xen0 causes the network to hang immediately.
> Ping >mtu from xen0 to xenU sometimes causes the network to hang, but
> not always. 'ping -i 0.1 -s 6000 <xenU ip>' will mostly cause the hang
> in under 30 iterations.
> 
> The recovery time appears to be in the order of 60 seconds or so, with
a
> partial recovery and then relapse at about 30 seconds.
> 
> When I was thinking about this problem, I was imagining a deadlock
> condition where rapid back to back packets (eg a fragmented icmp
packet
> from ping or a fragmented udp packet from nfs) was causing a hang
until
> part of the deadlock timed itself out and the packets started flowing
> again. I couldn't see 1 packet causing a buffer exhaustion unless it
got
> itself into a loop where it kept retrying to send the second fragment
> and didn't free the buffer each time, but even then the buffer bug
would
> be a side effect.
> 
> The deadlock would have to be caused in the transmit from xenU to
xen0,
> and something about the difference between sending a ping and
responding
> to a ping is the difference between always causing a lockup and only
> sometimes causing a lockup.
> 
> Maybe we're seeing different manifestations of the same problem?
> 
> James
> 
> > -----Original Message-----
> > From: xen-devel-admin@xxxxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
> > admin@xxxxxxxxxxxxxxxxxxxxx] On Behalf Of Bin Ren
> > Sent: Thursday, 16 September 2004 09:54
> > To: xen-devel@xxxxxxxxxxxxxxxxxxxxx
> > Subject: Re: [Xen-devel] network hang trigger
> >
> > I've just modified the netfront.c (haven't touched netback.c yet).
> I've
> > done two modifications: (1) free sk_buff properly on the transmit
path
> (2)
> > In netif_poll(...) function, packets **should not** be passed to
> > netif_rx(); instead, use: int netif_receive_skb(struct sk_buff
*skb).
> >
> > With these two modifications, under 'ping -s 6000', network only
> > occasionally loses a few packets but *very soon* recovers. It's much
> more
> > stable than before. I'll take a closer look at netfront.c and
> netback.c
> > tomorrow.
> >
> > Here is the patch. Please try it out. With your results, the changes
> may
> > get pushed into the repository.
> >
> 
> 
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