svn commit: r289276 - in head/sys: conf kern netinet sys

[Hiren Panchasara]

On 10/13/15 at 06:58P, Bryan Drewery wrote:
> On 10/13/2015 5:35 PM, Hiren Panchasara wrote:
> > Author: hiren
> > Date: Wed Oct 14 00:35:37 2015
> > New Revision: 289276
> > URL: https://svnweb.freebsd.org/changeset/base/289276
> > 
> > Log:
> >   There are times when it would be really nice to have a record of the last few
> >   packets and/or state transitions from each TCP socket. That would help with
> >   narrowing down certain problems we see in the field that are hard to reproduce
> >   without understanding the history of how we got into a certain state. This
> >   change provides just that.
> >   
> >   It saves copies of the last N packets in a list in the tcpcb. When the tcpcb is
> >   destroyed, the list is freed. I thought this was likely to be more
> >   performance-friendly than saving copies of the tcpcb. Plus, with the packets,
> >   you should be able to reverse-engineer what happened to the tcpcb.
> >   
> >   To enable the feature, you will need to compile a kernel with the TCPPCAP
> >   option. Even then, the feature defaults to being deactivated. You can activate
> >   it by setting a positive value for the number of captured packets. You can do
> >   that on either a global basis or on a per-socket basis (via a setsockopt call).
> >   
> >   There is no way to get the packets out of the kernel other than using kmem or
> >   getting a coredump. I thought that would help some of the legal/privacy concerns
> >   regarding such a feature. However, it should be possible to add a future effort
> >   to export them in PCAP format.
> >   
> >   I tested this at low scale, and found that there were no mbuf leaks and the peak
> >   mbuf usage appeared to be unchanged with and without the feature.
> >   
> >   The main performance concern I can envision is the number of mbufs that would be
> >   used on systems with a large number of sockets. If you save five packets per
> >   direction per socket and have 3,000 sockets, that will consume at least 30,000
> >   mbufs just to keep these packets. I tried to reduce the concerns associated with
> >   this by limiting the number of clusters (not mbufs) that could be used for this
> >   feature. Again, in my testing, that appears to work correctly.
> >   
> >   Differential Revision:	D3100
> 
> You're supposed to use the full URL here which will auto close the review.

Okay. It did pick up the commit though. What more does it need to know? 
> 
> I also replied to the review with style findings just now.
>
I'll ask Jonathan to look at it.

Cheers,
Hiren
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