netdev - Re: [RFC net-next PATCH V3 0/2] qdisc bulk dequeuing and utilizing delayed tailptr updates

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Date:	Fri, 19 Sep 2014 15:52:55 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <cwang@...pensource.com>
Cc:	Jesper Dangaard Brouer <brouer@...hat.com>,
	netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Tom Herbert <therbert@...gle.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Florian Westphal <fw@...len.de>,
	Daniel Borkmann <dborkman@...hat.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Alexander Duyck <alexander.duyck@...il.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Dave Taht <dave.taht@...il.com>, toke@...e.dk
Subject: Re: [RFC net-next PATCH V3 0/2] qdisc bulk dequeuing and utilizing
 delayed tailptr updates

On Fri, 2014-09-19 at 15:39 -0700, Cong Wang wrote:

> Then why commits like this one?
> 
> commit 7070ce0a6419a118842298bc967061ad6cea40db
> Author: Alexander Duyck <alexander.h.duyck@...el.com>
> Date:   Sat Sep 28 06:00:37 2013 +0000
> 
>     i40e: Add support for Tx byte queue limits
> 
>     Implement BQL (byte queue limit) support in i40e.
> 

Read again my full mail, don't jump into conclusions in the middle of
it.


> 
> >
> > If a device driver do nothing about BQL, byte_queue_limits/limit will be
> > zero by default.

See here ? Its all explained. If a device driver do not implement BQL,
limit will be zero, and bulk dequeue will _automatically_ be disabled.

BQL has no 'cost' if a driver does not use it.

Its basically fields which will never be updated.

(cf in struct netdev_queue :
			struct dql      dql;
)

> 

> Also note that for some virtual device, BQL may make no sense at all,
> at least veth doesn't even use a TX queue (I know it has tx-0 mostly for
> qdisc). Perhaps all NETIF_F_LLTX ones?

Lets focus on the normal cases, where virtual devices do not have any
qdisc.

We are talking here of _qdisc_ bulk dequeue, right ?




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