netdev - Re: [RFC] net: remove busylock

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Date:	Fri, 20 May 2016 06:47:49 -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 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.