9.2 ixgbe tx queue hang

[Christopher Forgeron]

This is regarding the TSO patch that Rick suggested earlier. (With many
thanks for his time and suggestion)

As I mentioned earlier, it did not fix the issue on a 10.0 system. It did
make it less of a problem on 9.2, but either way, I think it's not needed,
and shouldn't be considered as a patch for testing/etc.

Patching TSO to anything other than a max value (and by default the code
gives it IP_MAXPACKET) is confusing the matter, as the packet length
ultimately needs to be adjusted for many things on the fly like TCP
Options, etc. Using static header sizes won't be a good idea.

Additionally, it seems that setting nic TSO will/may be ignored by code
like this in sys/netinet/tcp_output.c:

10.0 Code:

  780                         if (len > tp->t_tsomax - hdrlen)
{                 !!
  781                                 len = tp->t_tsomax -
hdrlen;               !!
  782                                 sendalot =
1;
  783                         }


I've put debugging here, set the nic's max TSO as per Rick's patch ( set to
say 32k), and have seen that tp->t_tsomax == IP_MAXPACKET. It's being set
someplace else, and thus our attempts to set TSO on the nic may be in vain.

It may have mattered more in 9.2, as I see the code doesn't use
tp->t_tsomax in some locations, and may actually default to what the nic is
set to.

The NIC may still win, I didn't walk through the code to confirm, it was
enough to suggest to me that setting TSO wouldn't fix this issue.

However, this is still a TSO related issue, it's just not one related to
the setting of TSO's max size.

A 10.0-STABLE system with tso disabled on ix0 doesn't have a single packet
over IP_MAXPACKET in 1 hour of runtime. I'll let it go a bit longer to
increase confidence in this assertion, but I don't want to waste time on
this when I could be logging problem packets on a system with TSO enabled.

Comments are very welcome..