netdev - Re: [net-next PATCH V6 0/2] qdisc: bulk dequeue support

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Message-ID: <20141002165215.GJ1803@breakpoint.cc>
Date:	Thu, 2 Oct 2014 18:52:15 +0200
From:	Florian Westphal <fw@...len.de>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Jesper Dangaard Brouer <brouer@...hat.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.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 Høiland-Jørgensen <toke@...e.dk>
Subject: Re: [net-next PATCH V6 0/2] qdisc: bulk dequeue support

Tom Herbert <therbert@...gle.com> wrote:
> On Wed, Oct 1, 2014 at 1:35 PM, Jesper Dangaard Brouer
> <brouer@...hat.com> wrote:
> > This patchset uses DaveM's recent API changes to dev_hard_start_xmit(),
> > from the qdisc layer, to implement dequeue bulking.
> >
> > Patch01: "qdisc: bulk dequeue support for qdiscs with TCQ_F_ONETXQUEUE"
> >  - Implement basic qdisc dequeue bulking
> >  - This time, 100% relying on BQL limits, no magic safe-guard constants
> >
> > Patch02: "qdisc: dequeue bulking also pickup GSO/TSO packets"
> >  - Extend bulking to bulk several GSO/TSO packets
> >  - Seperate patch, as it introduce a small regression, see test section.
> >
> > We do have a patch03, which exports a userspace tunable as a BQL
> > tunable, that can byte-cap or disable the bulking/bursting.  But we
> > could not agree on it internally, thus not sending it now.  We
> > basically strive to avoid adding any new userspace tunable.
> >
> Unfortunately we probably still need something. If BQL were disabled
> (by setting BQL min_limit to infinity) then we'll always dequeue all
> the packets in the qdisc. Disabling BQL might be legitimate in
> deployment if say a bug is found in a device that prevents prompt
> transmit completions for some corner case.

Hmm.  Thats confusing.

So you are saying to disable bql one should do

cat limit_max > limit_min ?

But thats not the same as having a bql-unaware driver.

Seems to get same behavior as non-bql aware driver (where
dql_avail always returns 0 since num_queued and adj_limit remain at 0)
is to set

echo 0 > limit_max

... which makes dql_avail() return 0, which then also turns off bulk
dequeue.

Confused,
Florian
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