Re: [PATCH] sched: Optimize return value of qdisc_restart

[jamal <hadi@cyberus.ca>]

On Wed, 2007-09-05 at 01:12 -0700, David Miller wrote:

> Something this evening is obviously making it impossible
> for my brain to understand this function and your patch,
> so I'm going to sleep on it and try again tomorrow :-)

It is one of those areas that are hard to size-up in a blink;->
Gut-feeling: It doesnt sit right with me as well.
 
With (2.6.18-rxX++) QDISC_RUNNING changes that mean only one of N CPUs
will be dequeueing while the N-1 maybe enqueueing concurently. All N
CPUs contend for the queue lock; and theres a possible window between
releasing the queue lock by the dequeuer-CPU and enqueuer-CPU for a
race. The dequeuer-CPU entering one last time helps.

Krishna, you probably saw this "wasted entry into qdisc" under low
traffic conditions with more than likely only one CPU sending, am i
correct? Under heavier traffic when we have multiple CPUs funneling to
the same device, that entry is not really a "waste" because we endup
only go in once per X number of packets enqueued on the qdisc and that
check is absolutely necessary because a different CPU may have enqueued
while you were not looking. In the case of low traffic, X=1 - so it is a
waste there albeit a necessary one.

cheers,
jamal