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

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Date:	Fri, 16 Dec 2011 10:28:16 -0800
From:	Jesse Brandeburg <jesse.brandeburg@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Rick Jones <rick.jones2@...com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Vijay Subramanian <subramanian.vijay@...il.com>,
	tcpdump-workers@...ts.tcpdump.org, netdev@...r.kernel.org,
	Matthew Vick <matthew.vick@...el.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Subject: Re: twice past the taps, thence out to net?

On Thu, Dec 15, 2011 at 8:27 PM, Eric Dumazet <eric.dumazet@...il.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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