netdev - Re: [PATCH RFC] sched: split classification and enqueue

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Message-ID: <CAADnVQJzcbjiB3FPMcuHOftreL3reu+AZyLWeX_et=3_X8_bcw@mail.gmail.com>
Date:	Wed, 22 Jun 2016 10:29:44 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Florian Westphal <fw@...len.de>,
	john fastabend <john.fastabend@...il.com>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH RFC] sched: split classification and enqueue

On Wed, Jun 22, 2016 at 3:03 AM, Florian Westphal <fw@...len.de> wrote:
> Currently classification and enqueue is done in a single step.
>
> core acquires the qdisc lock, then calls the ->enqueue() function
> of the qdisc.
>
> Its the job of the qdisc and its attached classifiers to figure out what
> to do next.
>
> Typically the enqueue function will call tc_classify() to lookup a
> child class, then call ->enqueue of the child qdisc.
>
> This can repeat a number of times until a leaf qdisc is reached; this leaf
> will do the real enqueue operation (pfifo for example).
>
> While this approach gives qdiscs and the classifier/action subsystem
> a lot of control, it has one major drawback:  The root qdisc lock
> is held for a prolonged period of time while we recurse through
> the qdisc hierarchy from root to leaf.
>
> This (unfinished!) hack splits classification and enqueue into
> two steps.
>
> Before enqueueing the packet and *before* acquiring the root qdisc lock,
> the new qdisc ->classify() function is invoked.

I believe John is finalizing his lockless qdisc patches...
would this split still be needed after qdiscs become lockless?