Re: Socket buffer sizes with autotuning

[David Miller <davem@davemloft.net>]

From: "Jerry Chu" <hkchu@google.com>
Date: Mon, 12 May 2008 15:22:55 -0700

> I did a quick prototype based on your idea of adding an "in_flight"
> field to skb_shared_info to track how many in-flight clones in the
> host. I tested
> it quickly and it doesn't work. After some thought it was obvious why it
> won't work. It's because what the TCP stack needs is to track how
> many in-flight pkts are in the host, but your proposed patch increments
> "in_flight" once on the 1st __skb_clone() to be sent to the driver, but
> decrements "in_flight" TWICE, one for each of the clones to be freed.
> I did a quick hack to make it work for my limited test case but I haven't
> figured out an acceptable (non-hack) solution.

That's easy to fix, only set the in_flight pointer in the child clone
skb.  Thanks for figuring that out.

> Continued testing, I discovered the problem I described below where
> "in_flight" may point to a tp that has already been freed can not be
> addressed by zapping skb_shinfo(skb)->in_flight in sock_wfree(). The
> reason is that pkts may be acked and freed by TCP before driver freeing
> up its clone copy (e.g., due to driver lazy reclaim...) When that happens
> the "host_inflight" accounting will get messed up.

Simply notice, when we're about to decrement in_flight, that the data
reference is one.  You can take appropriate actions if so.