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@redhat.com>
Acked-by: Eric Dumazet <edumazet@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?

cheers