Re: root_lock vs. device's TX lock

[Tom Herbert <therbert@xxxxxxxxxx>]

> Actually, I'm interested in circumventing *both* locks. Our SoC has
> some quite-versatile queueing infrastructure, such that (for many
> queueing setups) we can do all of the queueing in hardware, using
> per-cpu access portals. By hacking around the qdisc lock, and using a
> tx queue per core, we were able to achieve a significant speedup.
>
This was actually one of the motivations for my question.  If we have
a one TX queue per core, and and use a trivial mq aware qdisc for
instance, the locking becomes mostly overhead.  I don't mind taking a
lock once per TX, but right now were taking three! (root lock twice,
and device lock once).

Even without one queue per TX, I think the overhead savings may still
be present.  Eric, I realize that a point of dropping the root lock in
sch_direct_xmit is to possibly allow queuing to to qdisc and device
xmit in parallel, but if you're using a trivial qdisc then the time in
qdisc may be << time in device xmit, so the overhead of locking could
mitigate the gains in parallelism.  At the very least, this benefit is
hugely variable depending on the qdisc used.

Tom
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html