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    SubjectRE: [PATCH 1/2] NET: Multiple queue network device support
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    > -----Original Message-----
    > From: Jarek Poplawski [mailto:jarkao2@o2.pl]
    > Sent: Monday, March 12, 2007 1:58 AM
    > To: Thomas Graf
    > Cc: Kok, Auke-jan H; David Miller; Garzik, Jeff;
    > netdev@vger.kernel.org; linux-kernel@vger.kernel.org;
    > Waskiewicz Jr, Peter P; Brandeburg, Jesse; Kok, Auke; Ronciak, John
    > Subject: Re: [PATCH 1/2] NET: Multiple queue network device support
    >
    > On 09-03-2007 14:40, Thomas Graf wrote:
    > > * Kok, Auke <auke-jan.h.kok@intel.com> 2007-02-08 16:09
    > >> diff --git a/net/core/dev.c b/net/core/dev.c index
    > 455d589..42b635c
    > >> 100644
    > >> --- a/net/core/dev.c
    > >> +++ b/net/core/dev.c
    > >> @@ -1477,6 +1477,49 @@ gso:
    > >> skb->tc_verd = SET_TC_AT(skb->tc_verd,AT_EGRESS);
    > >> #endif
    > >> if (q->enqueue) {
    > >> +#ifdef CONFIG_NET_MULTI_QUEUE_DEVICE
    > >> + int queue_index;
    > >> + /* If we're a multi-queue device, get a queue
    > index to lock */
    > >> + if (netif_is_multiqueue(dev))
    > >> + {
    > >> + /* Get the queue index and lock it. */
    > >> + if (likely(q->ops->map_queue)) {
    > >> + queue_index = q->ops->map_queue(skb, q);
    > >> +
    > spin_lock(&dev->egress_subqueue[queue_index].queue_lock);
    > >> + rc = q->enqueue(skb, q);
    >
    > I'm not sure Dave Miller thought about this place, when he
    > proposed to save the mapping, but I think this could be not
    > enough. This place is racy: ->map_queue() is called 2 times
    > and with some filters (and
    > policies/actions) results could differ. And of course the
    > subqueue lock doesn't prevent any filter from a config change
    > in the meantime.
    >
    > After second reading of this patch I have doubts it's the
    > proper way to solve the problem: there are many subqueues but
    > we need a top queue (prio here) to mange them, anyway. So,
    > why not to build this functionality directly into the queue?
    > There is no difference for a device if skbs are going from
    > the subqueue or a class, it is only interested in the mapping
    > result and a possibility to stop and start a subqueue and to
    > query its status. All this could be done by adding the
    > callbacks directly to any classful scheduler or, if not
    > enough, to write some specialized qdisc based on prio. The
    > possibility to lock only a subqueue instead of a queue could
    > be analized independently - current proposal doesn't solve
    > this anyway.
    >
    > Regards,
    > Jarek P.
    >

    Thanks again for the feedback. Given some discussions I had last week
    in the office and the general feedback here, I'm going to remove the new
    per-queue locking and leave the start/stop functions for each queue and
    combine entry points for hard_start_xmit(). I'll get this out asap for
    review once it's been tested here. If we see issues in the future with
    lock contention on the queues, we can revisit the per-queue locking.

    Cheers,
    -PJ Waskiewicz
    -
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