Date: Mon, 24 Mar 2014 17:14:08 +0100 From: Markus Gebert <markus.gebert@hostpoint.ch> To: Christopher Forgeron <csforgeron@gmail.com> Cc: FreeBSD Net <freebsd-net@freebsd.org>, Rick Macklem <rmacklem@uoguelph.ca>, Garrett Wollman <wollman@freebsd.org>, Jack Vogel <jfvogel@gmail.com> Subject: Re: 9.2 ixgbe tx queue hang Message-ID: <0BC10908-2081-45AC-A1C8-14220D81EC0A@hostpoint.ch> In-Reply-To: <CAB2_NwDKkgTfNuapm2gA5xhuBgVK6jE2uHwb2Nu-vsRvw_NwKQ@mail.gmail.com> References: <CAB2_NwAcDPM6YKNLQMC0=YSp%2Bn9nBpXGJQR9ajbgbfcQFoWYPw@mail.gmail.com> <1164414873.1690348.1395622026185.JavaMail.root@uoguelph.ca> <CAB2_NwAbHzFqa8RM5pwV7Yy5t=96JwzaF%2BSdjJN9kK3uhKKn_w@mail.gmail.com> <CAB2_NwCHM9D1HZSMsuQQ-dYNAt-t2721jKqfO=2h3M4qdumY7w@mail.gmail.com> <CAB2_NwDKkgTfNuapm2gA5xhuBgVK6jE2uHwb2Nu-vsRvw_NwKQ@mail.gmail.com>
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On 24.03.2014, at 16:21, Christopher Forgeron <csforgeron@gmail.com> =
wrote:
> This is regarding the TSO patch that Rick suggested earlier. (With =
many
> thanks for his time and suggestion)
>=20
> 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.
>=20
> 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.
>=20
> Additionally, it seems that setting nic TSO will/may be ignored by =
code
> like this in sys/netinet/tcp_output.c:
>=20
> 10.0 Code:
>=20
> 780 if (len > tp->t_tsomax - hdrlen)
> { !!
> 781 len =3D tp->t_tsomax -
> hdrlen; !!
> 782 sendalot =3D
> 1;
> 783 }
>=20
>=20
> 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 =3D=3D IP_MAXPACKET. It's =
being set
> someplace else, and thus our attempts to set TSO on the nic may be in =
vain.
>=20
> 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.
>=20
> 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=92s 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=92s tcp_mss() function. Some subfunctions get called:
tcp_mss() -> tcp_mss_update() -> tcp_maxmtu()
Then tcp_maxmtu() indeed uses the interface=92s hw_tsomax value:
1746 cap->tsomax =3D ifp->if_hw_tsomax;
It get=92s 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=92ll 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.
>=20
> 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.
>=20
> Comments are very welcome..
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>=20
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