On 24.03.2014, at 16:21, Christopher Forgeron <csforge...@gmail.com> wrote:
> 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.
I just applied Rick’s ixgbe TSO patch and additionally wanted to be able to
easily change the value of hw_tsomax, so I made a sysctl out of it.
While doing that, I asked myself the same question. Where and how will this
value actually be used and how comes that tcp_output() uses that other value in
struct tcpcb.
The only place tcpcb->t_tsomax gets set, that I have found so far, is in
tcp_input.c’s tcp_mss() function. Some subfunctions get called:
tcp_mss() -> tcp_mss_update() -> tcp_maxmtu()
Then tcp_maxmtu() indeed uses the interface’s hw_tsomax value:
1746 cap->tsomax = ifp->if_hw_tsomax;
It get’s passed back to tcp_mss() where it is set on the connection level
which will be used in tcp_output() later on.
tcp_mss() gets called from multiple places, I’ll look into that later. I will
let you know if I find out more.
Markus
> 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..
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