Re: twice past the taps, thence out to net?

[Jesse Brandeburg <jesse.brandeburg () gmail com>]

On Thu, Dec 15, 2011 at 8:27 PM, Eric Dumazet <eric.dumazet () gmail com> wrote:
> Le jeudi 15 décembre 2011 à 14:22 -0800, Rick Jones a écrit :
> > On 12/15/2011 11:00 AM, Eric Dumazet wrote:
> > > > Device's work better if the driver proactively manages stop_queue/wake_queue.
> > > > Old devices used TX_BUSY, but newer devices tend to manage the queue
> > > > themselves.
> > >
> > > Some 'new' drivers like igb can be fooled in case skb is gso segmented ?
> > >
> > > Because igb_xmit_frame_ring() needs skb_shinfo(skb)->nr_frags + 4
> > > descriptors, igb should stop its queue not at MAX_SKB_FRAGS + 4, but
> > > MAX_SKB_FRAGS*4

can you please help me understand the need for MAX_SKB_FRAGS * 4 as
the requirement?  Currently driver uses logic like

in hard_start_tx: hey I just finished a tx, I should stop the qdisc if
I don't have room (in tx descriptors) for a worst case transmit skb
(MAX_SKB_FRAGS + 4) the next time I'm called.
when cleaning from interrupt: My cleanup is done, do I have enough
free tx descriptors (should be MAX_SKB_FRAGS + 4) for a worst case
transmit?  If yes, restart qdisc.

I'm missing the jump from the above logic to using MAX_SKB_FRAGS * 4
(== (18 * 4) == 72) as the minimum number of descriptors I need for a
worst case TSO.  Each descriptor can point to up to 16kB of contiguous
memory, typically we use 1 for offload context setup, 1 for skb->data,
and 1 for each page.  I think we may be overestimating with
MAX_SKB_FRAGS + 4, but that should be no big deal.
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