Message-ID: <20080919185150.GB2888@ami.dom.local>
Date:	Fri, 19 Sep 2008 20:51:50 +0200
From:	Jarek Poplawski <jarkao2@...il.com>
To:	"Duyck, Alexander H" <alexander.h.duyck@...el.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"kaber@...sh.net" <kaber@...sh.net>
Subject: Re: [RFC PATCH] sched: only dequeue if packet can be queued to
	hardware queue.

On Fri, Sep 19, 2008 at 11:01:12AM -0700, Duyck, Alexander H wrote:
> Jarek Poplawski wrote:
> 
> >> Also changing dequeue_skb will likely cause additional issues for
> >> several qdiscs as it doesn't report anything up to parent queues, as
> >> a result you will end up with qdiscs like prio acting more like
> >> multiq because they won't know if a queue is empty, stopped, or
> >> throttled.
> >> Also I believe this will cause a panic on pfifo_fast and several
> >> other qdiscs because some check to see if there are packets in the
> >> queue and if so dequeue with the assumption that they will get a
> >> packet out.  You will need to add checks for this to resolve this
> >> issue.
> >
> > I really can't get your point. Don't you mean skb_dequeue()?
> > dequeue_skb() is used only by qdisc_restart()...
> You're right.  I misread it as skb_dequeue.  The problem is though you
> are still relying on q->requeue which last I knew was only being used
> a few qdiscs.  In addition you will still be taking the cpu hit for the
> dequeue/requeue on several qdiscs which can't use q->requeue without
> violating the way they were supposed to work.

I'm not sure I understand your point. No qdisc will use any new
q->requeue. All qdisc do dequeue, but on tx fail an skb isn't requeued
back, but queued by qdisc_restart() in a private list (of root qdisc),
and then tried before any new dequeuing. There will be no cpu hit,
because each next try is only skb_peek() on this list, until tx queue
of an skb at the head is working.

> >> The one thing I liked about my approach was that after I was done you
> >> could have prio as a parent of multiq leaves or multiq as the parent
> >> of prio leaves and all the hardware queues would receive the packets
> >> in the same order as the result.
> >
> > I'm not against your approach, but I'd like to be sure these
> > complications are really worth of it. Of course, if my proposal, the
> > first take of 3 patches, doesn't work as you predict (and I doubt),
> > then we can forget about it.
> Well when you get some testing done let me know.  The main areas I am
> concerned with are:
> 
> 1.  CPU utilization stays the same regardless of which queue used.
> 2.  Maintain current qdisc behavior on a per hw queue basis.
> 3.  Avoid head-of-line blocking where it applies
>         for example: prio band 0 not blocked by band 1, or 1 by 2, etc..
>                                 or
>                          multiq not blocked on any band due to 1 band blocked
> 
> As long as all 3 criteria are met I would be happy with any solution
> provided.

Alas this testing is both the weekest (I'm not going to do this), and
the strongest side of this solution, because it's mostly David's work
(my patch could be actually skipped without much impact). So, I hope,
you don't suggest this could be not enough tested or otherwise not the
best?!

Anyway, I don't insist on "my" solution. I simply think it's reasonable,
and it's not for private reasons, because I didn't even find this out.
I think, if it's so wrong you should have no problem to show this in
one little test. And maybe David changed his mind in the meantime and he
will choose your way even without this.

Thanks,
Jarek P.
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