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

[David Ahern <dsahern@kernel.org>]

On 12/8/23 12:22 PM, Mina Almasry wrote:
> On Fri, Dec 8, 2023 at 9:48 AM David Ahern <dsahern@kernel.org> 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?

I think it depends on the details of how entries are added and removed
from the pool. I am behind on the pp details at this point, so I do need
to do some homework.

> 
> 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?

It has to do with the usability of this overall solution. As I mentioned
most ML use cases can (and will want to) use many memory allocations for
receiving packets - e.g., allocations per message and receiving multiple
messages per socket connection.

> 
>>> +             }
>>> +     }
>>> +
>>> +     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.

I am not referring to 1:N or N:1 for page pool and queues. I am
referring to entries within a single page pool for a single Rx queue.


> 
> 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.
>