DNS (?) not working on G5 (64-bit powerpc) (was [net-next,v3,3/3] udp: try to avoid 2 cache miss on dequeue)

[Paolo Abeni]

On Thu, 2017-06-22 at 18:43 +0200, Paolo Abeni wrote:
> On Thu, 2017-06-22 at 23:06 +1000, Michael Ellerman wrote:
> > Paolo wrote:
> > > when udp_recvmsg() is executed, on x86_64 and other archs, most skb
> > > fields are on cold cachelines.
> > > If the skb are linear and the kernel don't need to compute the udp
> > > csum, only a handful of skb fields are required by udp_recvmsg().
> > > Since we already use skb->dev_scratch to cache hot data, and
> > > there are 32 bits unused on 64 bit archs, use such field to cache
> > > as much data as we can, and try to prefetch on dequeue the relevant
> > > fields that are left out.
> > > 
> > > This can save up to 2 cache miss per packet.
> > > 
> > > v1 -> v2:
> > >   - changed udp_dev_scratch fields types to u{32,16} variant,
> > >     replaced bitfiled with bool
> > > 
> > > Signed-off-by: Paolo Abeni <pabeni at redhat.com>
> > > Acked-by: Eric Dumazet <edumazet at google.com>
> > > ---
> > >  net/ipv4/udp.c | 114 +++++++++++++++++++++++++++++++++++++++++++++++++++------
> > >  1 file changed, 103 insertions(+), 11 deletions(-)
> > 
> > This appears to break wget on one of my machines.
> > 
> > Networking in general is working, I'm able to SSH in, but then I can't
> > do a wget.
> > 
> > eg:
> > 
> > $ wget google.com
> > --2017-06-22 22:45:39--  http://google.com/
> > Resolving proxy.pmdw.com (proxy.pmdw.com)... failed: Temporary failure in name resolution.
> > wget: unable to resolve host address ‘proxy.pmdw.com’
> > 
> > $ host proxy.pmdw.com
> > proxy.pmdw.com is an alias for raven.pmdw.com.
> > raven.pmdw.com has address 10.1.2.3
> > 
> > $ wget google.com
> > --2017-06-22 22:52:08--  http://google.com/
> > Resolving proxy.pmdw.com (proxy.pmdw.com)... failed: Temporary failure in name resolution.
> > wget: unable to resolve host address ‘proxy.pmdw.com’
> > 
> > Maybe host is using TCP but the man page says it doesn't?
> > 
> > 
> > Everything is OK if I boot back to the previous commit 0a463c78d25b
> > ("udp: avoid a cache miss on dequeue"):
> > 
> > $ wget google.com
> > --2017-06-22 23:00:01--  http://google.com/
> > Resolving proxy.pmdw.com (proxy.pmdw.com)... 10.1.2.3
> > Connecting to proxy.pmdw.com (proxy.pmdw.com)|10.1.2.3|:3128... connected.
> > Proxy request sent, awaiting response... 302 Found
> > Location: http://www.google.com.au/?gfe_rd=cr&ei=Ub9LWbPbLujDXrH1uPgE [following]
> > --2017-06-22 23:00:01--  http://www.google.com.au/?gfe_rd=cr&ei=Ub9LWbPbLujDXrH1uPgE
> > Reusing existing connection to proxy.pmdw.com:3128.
> > Proxy request sent, awaiting response... 200 OK
> > Length: unspecified [text/html]
> > Saving to: ‘index.html’
> > 
> > index.html                      [ <=>                                       ]  11.37K  --.-KB/s    in 0.001s  
> > 
> > 2017-06-22 23:00:01 (22.0 MB/s) - ‘index.html’ saved [11640]
> > 
> > $ uname -a
> > Linux 4.12.0-rc4-gcc6-00988-g0a463c7 #88 SMP Thu Jun 22 22:55:12 AEST 2017 ppc64 GNU/Linux
> > 
> > 
> > Haven't had time to debug any further. Any ideas?
> 
> Thank you for this report.
> 
> Can you please specify features of the relevant NIC ? (ethtool -k
> <name>) 
> 
> I'll try to replicate the issue as soon I'll get hands on suitable HW,

I had my hands on power7, but I can't trivially reproduce the issue so
I'm going to bug you for more info. 

Can you please specify the host CPU, the NIC in use (ethtool -i
<name>), the compiler version used to build the kernel and possibly
provide a tcpdump of the DNS packets received/sent while running wget
and while running the host command?

Do you have the relevant kernel running on others PPC hosts?

Thank you,

Paolo