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SubjectRe: 2.6.25-rc6-git2: warn_on_slowpath for tcp_simple_retransmit
On Wed, 2 Apr 2008, Alessandro Suardi wrote:

> On Wed, Apr 2, 2008 at 10:10 AM, Ilpo Järvinen
> <ilpo.jarvinen@helsinki.fi> wrote:
> > On Wed, 2 Apr 2008, Alessandro Suardi wrote:
> >
> > > Found this in my FC6-based bittorrent box (K7-800 running
> > > a 2.6.25-rc6-git2 kernel) this evening.
> > >
> > > The kernel was upgraded two weeks ago to fix the bug
> > > in which an USB VIA driver hammered the PCI bus
> > > causing ATA disk performance to drop, and has been
> > > running since then (it still is).
> > >
> > > So it's actually 2.6.25-rc6-git2 plus patch as in here
> > > http://www.gossamer-threads.com/lists/linux/kernel/895506#895506
> > >
> > > If there's anything useful I can do, just ask. Thanks !
> >
> > Can you reproduce?
>
> Nope. That only happened once in this uptime:
>
> [root@donkey ~]# uptime
> 21:57:21 up 14 days, 22:04, 5 users, load average: 0.64, 0.47, 0.44
>
> The machine runs unattended as a bittorrent client, 24x7,
> with a very low traffic (uploading at a steady ~36KB/s,
> and downloads happen in peaks).
>
> I VNC into it in the evening and manage the torrents;
> that is all.
>
> Now, there's an interesting tidbit:
>
> [root@donkey ~]# ip -s link show eth0
> 3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
> link/ether 00:c0:49:a7:33:fe brd ff:ff:ff:ff:ff:ff
> RX: bytes packets errors dropped overrun mcast
> 959044156 60549131 0 0 0 0
> TX: bytes packets errors dropped carrier collsns
> 744533957 66149681 0 0 0 0
>
> [root@donkey ~]# ifconfig eth0
> eth0 Link encap:Ethernet HWaddr 00:C0:49:A7:33:FE
> inet addr:192.168.1.7 Bcast:192.168.1.255 Mask:255.255.255.0
> inet6 addr: fe80::2c0:49ff:fea7:33fe/64 Scope:Link
> UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
> RX packets:60551468 errors:0 dropped:0 overruns:0 frame:0
> TX packets:66152721 errors:0 dropped:0 overruns:1 carrier:0
> collisions:0 txqueuelen:1000
> RX bytes:959199592 (914.7 MiB) TX bytes:748395356 (713.7 MiB)
> Interrupt:12 Base address:0xce00
>
> Somehow the "overruns" counter is seen differently by 'ip'
> and 'ifconfig' - one says 0, the other says 1 - perhaps the
> packet that WARN'd me on tcp_simple_retransmit ?

...I find that extreme unlikely.

> If there's anything else - reproducing seems really really
> unlikely - 1 packet in 66 million...

Yeah, it seems some hard to hit corner case and so far nobody has
a reproducable scenario (or at least I'm not aware of any). I tried
to reproduce it last weekend and failed, even with some netem stimuli
added while torrenting.

I'll probably ask soon Andrew to queue some low cost debug patch into
-mm to get a bit more clues when somebody running mm happens to hit
it.

--
i.
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