Re: [net-next v1 06/16] netdev: support binding dma-buf to netdevice

[Mina Almasry]

On Fri, Dec 8, 2023 at 11:22 AM Mina Almasry <almasrymina@xxxxxxxxxx> wrote:
>
> On Fri, Dec 8, 2023 at 9:48 AM David Ahern <dsahern@xxxxxxxxxx> wrote:
> >
> > On 12/7/23 5:52 PM, Mina Almasry wrote:
> ...
> > > +
> > > +     xa_for_each(&binding->bound_rxq_list, xa_idx, rxq) {
> > > +             if (rxq->binding == binding) {
> > > +                     /* We hold the rtnl_lock while binding/unbinding
> > > +                      * dma-buf, so we can't race with another thread that
> > > +                      * is also modifying this value. However, the driver
> > > +                      * may read this config while it's creating its
> > > +                      * rx-queues. WRITE_ONCE() here to match the
> > > +                      * READ_ONCE() in the driver.
> > > +                      */
> > > +                     WRITE_ONCE(rxq->binding, NULL);
> > > +
> > > +                     rxq_idx = get_netdev_rx_queue_index(rxq);
> > > +
> > > +                     netdev_restart_rx_queue(binding->dev, rxq_idx);
> >
> > Blindly restarting a queue when a dmabuf is heavy handed. If the dmabuf
> > has no outstanding references (ie., no references in the RxQ), then no
> > restart is needed.
> >
>
> I think I need to stop the queue while binding to a dmabuf for the
> sake of concurrency, no? I.e. the softirq thread may be delivering a
> packet, and in parallel a separate thread holds rtnl_lock and tries to
> bind the dma-buf. At that point the page_pool recreation will race
> with the driver doing page_pool_alloc_page(). I don't think I can
> insert a lock to handle this into the rx fast path, no?
>
> Also, this sounds like it requires (lots of) more changes. The
> page_pool + driver need to report how many pending references there
> are (with locking so we don't race with incoming packets), and have
> them reported via an ndo so that we can skip restarting the queue.
> Implementing the changes in to a huge issue but handling the
> concurrency may be a genuine blocker. Not sure it's worth the upside
> of not restarting the single rx queue?
>
> > > +             }
> > > +     }
> > > +
> > > +     xa_erase(&netdev_dmabuf_bindings, binding->id);
> > > +
> > > +     netdev_dmabuf_binding_put(binding);
> > > +}
> > > +
> > > +int netdev_bind_dmabuf_to_queue(struct net_device *dev, u32 rxq_idx,
> > > +                             struct netdev_dmabuf_binding *binding)
> > > +{
> > > +     struct netdev_rx_queue *rxq;
> > > +     u32 xa_idx;
> > > +     int err;
> > > +
> > > +     rxq = __netif_get_rx_queue(dev, rxq_idx);
> > > +
> > > +     if (rxq->binding)
> > > +             return -EEXIST;
> > > +
> > > +     err = xa_alloc(&binding->bound_rxq_list, &xa_idx, rxq, xa_limit_32b,
> > > +                    GFP_KERNEL);
> > > +     if (err)
> > > +             return err;
> > > +
> > > +     /* We hold the rtnl_lock while binding/unbinding dma-buf, so we can't
> > > +      * race with another thread that is also modifying this value. However,
> > > +      * the driver may read this config while it's creating its * rx-queues.
> > > +      * WRITE_ONCE() here to match the READ_ONCE() in the driver.
> > > +      */
> > > +     WRITE_ONCE(rxq->binding, binding);
> > > +
> > > +     err = netdev_restart_rx_queue(dev, rxq_idx);
> >
> > Similarly, here binding a dmabuf to a queue. I was expecting the dmabuf
> > binding to add entries to the page pool for the queue.
>
> To be honest, I think maybe there's a slight disconnect between how
> you think the page_pool works, and my primitive understanding of how
> it works. Today, I see a 1:1 mapping between rx-queue and page_pool in
> the code. I don't see 1:many or many:1 mappings.
>
> In theory mapping 1 rx-queue to n page_pools is trivial: the driver
> can call page_pool_create() multiple times to generate n queues and
> decide for incoming packets which one to use.
>
> However, mapping n rx-queues to 1 page_pool seems like a can of worms.
> I see code in the page_pool that looks to me (and Willem) like it's
> safe only because the page_pool is used from the same napi context.
> with a n rx-queueue: 1 page_pool mapping, that is no longer true, no?
> There is a tail end of issues to resolve to be able to map 1 page_pool
> to n queues as I understand and even if resolved I'm not sure the
> maintainers are interested in taking the code.
>
> So, per my humble understanding there is no such thing as "add entries
> to the page pool for the (specific) queue", the page_pool is always
> used by 1 queue.
>
> Note that even though this limitation exists, we still support binding
> 1 dma-buf to multiple queues, because multiple page pools can use the
> same netdev_dmabuf_binding. I should add that to the docs.
>
> > If the pool was
> > previously empty, then maybe the queue needs to be "started" in the
> > sense of creating with h/w or just pushing buffers into the queue and
> > moving the pidx.
> >
> >
>
> I don't think it's enough to add buffers to the page_pool, no? The
> existing buffers in the page_pool (host mem) must be purged. I think
> maybe the queue needs to be stopped as well so that we don't race with
> incoming packets and end up with skbs with devmem and non-devmem frags
> (unless you're thinking it becomes a requirement to support that, I
> think things are complicated as-is and it's a good simplification).
> When we already purge the existing buffers & restart the queue, it's
> little effort to migrate this to become in line with Jakub's queue-api
> that he also wants to use for per-queue configuration & ndo_stop/open.
>

FWIW what i'm referring to with Jakub's queue-api is here:
https://lore.kernel.org/netdev/20230815171638.4c057dcd@xxxxxxxxxx/

I made some simplifications, vis-a-vis passing the queue idx for the
driver to extract the config from rather than the 'cfg' param Jakub
outlined, and again passed the queue idx instead of the 'queue info'
(the API currently assumes RX, and can be extended later for TX use
cases).
-- 
Thanks,
Mina