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

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Message-ID: <1312877173.2371.2.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Tue, 09 Aug 2011 10:06:13 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	shemminger@...tta.com, therbert@...gle.com, netdev@...r.kernel.org
Subject: Re: [RFC PATCH v2 0/9] bql: Byte Queue Limits

Le mardi 09 août 2011 à 00:41 -0700, David Miller a écrit :
> 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. :-)
> 

I agree, it permits a smooth transition to selected drivers, even some
non NAPI ones ;)

In some stress situations, NAPI never completes, so it would also mean
adding a dql call in this case...



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