netdev - Re: [RFC] net: remove busylock

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Date:	Fri, 20 May 2016 06:11:49 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	Alexander Duyck <alexander.duyck@...il.com>,
	netdev <netdev@...r.kernel.org>,
	Alexander Duyck <aduyck@...antis.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [RFC] net: remove busylock

On Fri, 2016-05-20 at 09:29 +0200, Jesper Dangaard Brouer wrote:

> The hole idea behind allowing bulk qdisc dequeue, was to mitigate this,
> by allowing dequeue to do more work, while holding the lock.
> 
> You mention HTB.  Notice HTB does not take advantage of bulk dequeue.
> Have you tried to enable/allow HTB to bulk dequeue?
> 

Well, __QDISC___STATE_RUNNING means exactly that : one cpu is dequeueing
many packets from the qdisc and tx them to the device.

It is generic for any kind of qdisc.

HTB bulk dequeue would have to call ->dequeue() mutiple times. If you do
this while holding qdisc spinlock, you block other cpus from doing
concurrent ->enqueue(), adding latencies (always the same trade off...)

HTB wont be anytime soon have separate protections for the ->enqueue()
and the ->dequeue(). Have you looked at this monster ? I did, many
times...

Note that I am working on a patch to transform __QDISC___STATE_RUNNING
to a seqcount do that we can grab stats without holding the qdisc lock.