netdev - Re: [RFC PATCH v2 0/9] bql: Byte Queue Limits

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Message-Id: <20110809.004123.2080217558650594315.davem@davemloft.net>
Date:	Tue, 09 Aug 2011 00:41:23 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...tta.com
Cc:	therbert@...gle.com, netdev@...r.kernel.org
Subject: Re: [RFC PATCH v2 0/9] bql: Byte Queue Limits

From: Stephen Hemminger <shemminger@...tta.com>
Date: Mon, 8 Aug 2011 11:19:17 -0700

> On Mon, 8 Aug 2011 11:01:57 -0700
> Tom Herbert <therbert@...gle.com> wrote:
> 
>> > Since transmit completion means calling dev_kfree_skb() why not account
>> > there? You could add some info to netdev if necessary to get compile
>> > the statistics.
>> >
>> The algorithm depends on knowing the total number of packets competed
>> in a single execution of transmit completion (epic based).  We only
>> want to recalculate the limits once per completion, which happens when
>> the completion function is called.
> 
> So just add some stats to netdev and count the number of dev_kfree_skb
> calls and do your work at napi complete.

No, I think what Tom's doing here is reasonable.

First off, drivers have never been required to do specific things if
they return success for ->ndo_start_xmit().  They can kfree_skb(skb)
and return NET_TX_SUCCESS, and that's fine.

This mean we can't use ->ndo_start_xmit() return values to drive the
BQL engine.

So we simply have a way for the driver to say when it really does
queue up a packet, and that might be useful for other things.

Second, you absolutely do not want to run the BQL engine on every
single dev_kfree_skb() call.  You want to batch it, and that's what
Tom's new interface does, and that is therefore quite reasonable
too.

In fact, I think the best part about this revision of the BQL patches
is the device driver interface. :-)

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