On Thursday, 15. January 2015 08:00:58 Eric Dumazet wrote:

Please apply this patch, and try to lower
/proc/sys/net/core/gro_max_frags and see if this makes a difference
(leaving GRO enabled)

(start with 7 and increase it, limit being 17)

Patch applied to 3.19-rc4+.

Results:
 7: hang
 8: hang
 9: hang
10: hang
11: hang
12: hang
13: hang
14: hang
15: hang
16: hang
17: hang

for the sake of completeness:
1: hang
2: hang
3: hang
4: hang
5: hang
6: hang

Regarding the test procedure: I stopped the download script on the client,
changed gro_max_frags and started the download again. No cable unplugging / 
reboot of the box in between. Just mentioning it to make sure it somehow 
does not affect what we actually wanted to test.

Additional tests have been done with gro_max_frags 1, 7 and 17:
- stop networking + unload e1000e -> restart -> download: hang

One thing I can say from the testing: The more I increase gro_max_frags,
the longer it takes to trigger it. I tried each setting below three times.
A value of 17 is really noticeable.

1: 3-8 seconds till hang
7: 7-10 seconds till hang
17: 23-26 seconds till hang

Thomas