Re: [RFC] [PATCH] Don't run __qdisc_run() on a stopped TX queue

[Jarek Poplawski <jarkao2@gmail.com>]

On Wed, Jul 29, 2009 at 07:11:34PM +0800, Herbert Xu wrote:
> On Wed, Jul 29, 2009 at 11:04:36AM +0000, Jarek Poplawski wrote:
> >
> > How about this: instead of the _RUNNING flag we take tx lock while
> > holding qdisc lock and release qdisc lock just after (before xmit).
> > This should prevent reordering, and probably could improve cache use:
> > CPU B which takes qdisc lock only for enqueuing now, would use it for
> > dequeuing too, plus if accidentally the next xmit goes to a different
> > tx queue, it could start before CPU A finishes. Otherwise it would
> > simply wait for CPU A (without tx lock contention). Of course it
> > needs testing... 
> 
> Well reordering isn't the only problem, the lock contention brought
> upon by two CPUs both trying to transmit the same flow from the
> qdisc is just as bad.

If you mean the tx lock there should be no "real" contention: only
one waiter max. qdisc lock's contention might be higher, but it's
use (during contention) better: enqueue + dequeue together instead
of doing it separately.

Cheers,
Jarek P.