Message-ID: <606cd787d64da_22ba520855@john-XPS-13-9370.notmuch>
Date:   Tue, 06 Apr 2021 14:49:59 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Hangbin Liu <liuhangbin@...il.com>,
        John Fastabend <john.fastabend@...il.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Jiri Benc <jbenc@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        David Ahern <dsahern@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Björn Töpel <bjorn.topel@...il.com>
Subject: Re: [PATCHv4 bpf-next 2/4] xdp: extend xdp_redirect_map with
 broadcast support

Toke Høiland-Jørgensen wrote:
> Hangbin Liu <liuhangbin@...il.com> writes:
> 
> > On Mon, Apr 05, 2021 at 05:24:48PM -0700, John Fastabend wrote:
> >> Hangbin Liu wrote:
> >> > This patch add two flags BPF_F_BROADCAST and BPF_F_EXCLUDE_INGRESS to extend
> >> > xdp_redirect_map for broadcast support.
> >> > 
> >> > Keep the general data path in net/core/filter.c and the native data
> >> > path in kernel/bpf/devmap.c so we can use direct calls to get better
> >> > performace.
> >> > 
> >> > Here is the performance result by using xdp_redirect_{map, map_multi} in
> >> > sample/bpf and send pkts via pktgen cmd:
> >> > ./pktgen_sample03_burst_single_flow.sh -i eno1 -d $dst_ip -m $dst_mac -t 10 -s 64
> >> > 
> >> > There are some drop back as we need to loop the map and get each interface.
> >> > 
> >> > Version          | Test                                | Generic | Native
> >> > 5.12 rc2         | redirect_map        i40e->i40e      |    2.0M |  9.8M
> >> > 5.12 rc2         | redirect_map        i40e->veth      |    1.8M | 12.0M
> >> 
> >> Are these are 10gbps i40e ports? Sorry if I asked this earlier, maybe
> >> add a note in the commit if another respin is needed.
> >
> > Yes, I will add it if there is an update.
> >
> >> > diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> >> > index 3980fb3bfb09..c8452c5f40f8 100644
> >> > --- a/kernel/bpf/devmap.c
> >> > +++ b/kernel/bpf/devmap.c
> >> > @@ -198,6 +198,7 @@ static void dev_map_free(struct bpf_map *map)
> >> >  	list_del_rcu(&dtab->list);
> >> >  	spin_unlock(&dev_map_lock);
> >> >  
> >> > +	bpf_clear_redirect_map(map);
> >> 
> >> Is this a bugfix? If its needed here wouldn't we also need it in the
> >> devmap case.
> >
> > No, in ee75aef23afe ("bpf, xdp: Restructure redirect actions") this function
> > was removed. I added it back as we use ri->map again.
> >
> > What devmap case you mean?
> >
> >> 
> >> >  	synchronize_rcu();
> >> >  
> >> >  	/* Make sure prior __dev_map_entry_free() have completed. */
> >> 
> >> [...]
> >> 
> >> > +
> >> > +static struct bpf_dtab_netdev *devmap_get_next_obj(struct xdp_buff *xdp,
> >> > +						   struct bpf_map *map,
> >> > +						   u32 *key, u32 *next_key,
> >> > +						   int ex_ifindex)
> >> > +{
> >> > +	struct bpf_dtab_netdev *obj;
> >> > +	struct net_device *dev;
> >> > +	u32 index;
> >> > +	int err;
> >> > +
> >> > +	err = devmap_get_next_key(map, key, next_key);
> >> > +	if (err)
> >> > +		return NULL;
> >> > +
> >> > +	/* When using dev map hash, we could restart the hashtab traversal
> >> > +	 * in case the key has been updated/removed in the mean time.
> >> > +	 * So we may end up potentially looping due to traversal restarts
> >> > +	 * from first elem.
> >> > +	 *
> >> > +	 * Let's use map's max_entries to limit the loop number.
> >> > +	 */
> >> > +	for (index = 0; index < map->max_entries; index++) {
> >> > +		obj = devmap_lookup_elem(map, *next_key);
> >> > +		if (!obj || dst_dev_is_ingress(obj, ex_ifindex))
> >> > +			goto find_next;
> >> > +
> >> > +		dev = obj->dev;
> >> > +
> >> > +		if (!dev->netdev_ops->ndo_xdp_xmit)
> >> > +			goto find_next;
> >> > +
> >> > +		err = xdp_ok_fwd_dev(dev, xdp->data_end - xdp->data);
> >> > +		if (unlikely(err))
> >> > +			goto find_next;
> >> > +
> >> > +		return obj;
> >> > +
> >> > +find_next:
> >> > +		key = next_key;
> >> > +		err = devmap_get_next_key(map, key, next_key);
> >> > +		if (err)
> >> > +			break;
> >> > +	}
> >> 
> >> I'm missing something. Either an elaborated commit message or comment
> >> is probably needed. I've been looking at this block for 30 minutes and
> >> can't see how we avoid sending duplicate frames on a single interface?
> >> Can you check this code flow, 
> >> 
> >>   dev_map_enqueue_multi()
> >>    for (;;) {
> >>      next_obj = devmap_get_next_obj(...)
> >>         for (index = 0; index < map->max_entries; index++) {
> >>            obj = devmap_lookup_elem();
> >>            if (!obj) goto find_next
> >>            key = next_key;
> >>            err = devmap_get_next_key() 
> >>                   if (!key) goto find_first
> >>                   for (i = 0; i < dtab->n_buckets; i++)
> >>                      return *next <- now *next_key is point back
> >>                                      at first entry
> >>            // loop back through and find first obj and return that
> >
> > 	 devmap_get_next_key() will loop to find the first one if there is no
> > 	 key or dev. In normal time it will stop after the latest one.
> >>         }
> >>       bq_enqueue(...) // enqueue original obj
> >>       obj = next_obj;
> >>       key = next_key; 
> >>       ...  // we are going to enqueue first obj, but how do we know
> >>            // this hasn't already been sent? Presumably if we have
> >>            // a delete in the hash table in the middle of a multicast
> >>            // operation this might happen?
> >>    }
> >
> > And yes, there is an corner case that if we removed a dev during multicast,
> > there is an possibility that restart from the first key. But given that
> > this is an unlikely case, and in normal internet there is also a possibility
> > of duplicate/lost packet. This should also be acceptable?
> 
> In my mind this falls under "acceptable corner cases". I.e., if you're
> going to use the map for redirect and you expect to be updating it while
> you're doing so, don't use a hashmap. But if you will not be updating
> the map (or find the possible duplication acceptable), you can use the
> hashmap and gain the benefit of being able to index by ifindex.

In a Kubernetes setup its going to be hard, if possible at all, to restrict
the map from moving as interfaces/IPs are going to be dynamic. Using a
hash map has nice benefits of not having to figure out how to put ifindex's
into the array. Although on some early implementations I wrote a small
hashing algorithm over the top of array, so that could work.

I don't know how well multicast applications might handle duplicate packets.
I wouldn't be too surprised if it was problematic. On the other hand missing
an entry that was just added is likely OK. There is no way to know from
network/user side if the entry was actually added before multicast op and
skipped or insert happened just after multicast op. And vice versa for a
delete dev, no way to know the multicast op happened before/after the
delete.

Have we consider doing something like the batch lookup ops over hashtab?
I don't mind "missing" values so if we just walk the list?

     head = dev_map_index_hash(dtab, i)
     // collect all my devs and get ready to send multicast
     hlist_nulls_for_each_entry_safe(dev, next, head, index_hlist) {
		enqueue(dev, skb)
     }
     // submit the queue of entries and do all the work to actually xmit
     submit_enqueued();

We don't have to care about keys just walk the hash list?

> 
> But John does have a point that this is not obvious; so maybe it should
> be pointed out in the helper documentation?

At minimum it needs to be documented, but really I want to fix it. I
can see the confused end users sending me bug reports already about
duplicate packets.

> 
> -Toke
> 

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