netdev - Re: [RFC] net: remove busylock

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Date:	Fri, 20 May 2016 07:16:55 -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 06:47 -0700, Eric Dumazet wrote:
> On Fri, 2016-05-20 at 06:11 -0700, Eric Dumazet wrote:
> > 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.
> 
> Slide note : __qdisc_run() could probably avoid a __netif_schedule()
> when it breaks the loop, if another cpu is busy spinning on qdisc lock.
> 
> -> Less (spurious) TX softirq invocations, so less chance to trigger the
> infamous ksoftirqd bug we discussed lately.

Also note that in our case, we have HTB on a bonding device, and
FQ/pacing on slaves.

Since bonding pretends to be multiqueue, TCQ_F_ONETXQUEUE is not set
on sch->flags when HTB is installed at the bonding device root.

We might add a flag to tell qdisc layer that a device is virtual and
could benefit from bulk dequeue, since the ultimate TX queue is located
on another (physical) netdev, eventually MQ enabled.