netdev - RE: [RFC PATCH] sched: only dequeue if packet can be queued to hardware queue.

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Date:	Fri, 19 Sep 2008 11:01:12 -0700
From:	"Duyck, Alexander H" <alexander.h.duyck@...el.com>
To:	Jarek Poplawski <jarkao2@...il.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.

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.

>> 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.

Thanks,

Alex
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