Re: [PATCH v5 bpf-next 4/7] bpf: udp: Implement batching for sockets iterator

Aditi Ghag
2023-04-03T15:55:10Z

> On Mar 31, 2023, at 2:08 PM, Martin KaFai Lau <martin.lau@linux.dev> wrote:
>
> On 3/30/23 8:17 AM, Aditi Ghag wrote:
>> +static int bpf_iter_udp_realloc_batch(struct bpf_udp_iter_state *iter,
>> + unsigned int new_batch_sz);
>> static inline bool seq_sk_match(struct seq_file *seq, const struct sock *sk)
>> {
>> @@ -3151,6 +3163,149 @@ static inline bool seq_sk_match(struct seq_file *seq, const struct sock *sk)
>> net_eq(sock_net(sk), seq_file_net(seq)));
>> }
>> +static struct sock *bpf_iter_udp_batch(struct seq_file *seq)
>> +{
>> + struct bpf_udp_iter_state *iter = seq->private;
>> + struct udp_iter_state *state = &iter->state;
>> + struct net *net = seq_file_net(seq);
>> + struct sock *first_sk = NULL;
>> + struct udp_seq_afinfo afinfo;
>> + struct udp_table *udptable;
>> + unsigned int batch_sks = 0;
>> + bool resized = false;
>> + struct sock *sk;
>> + int offset = 0;
>> + int new_offset;
>> +
>> + /* The current batch is done, so advance the bucket. */
>> + if (iter->st_bucket_done) {
>> + state->bucket++;
>> + iter->offset = 0;
>> + }
>> +
>> + afinfo.family = AF_UNSPEC;
>> + afinfo.udp_table = NULL;
>> + udptable = udp_get_table_afinfo(&afinfo, net);
>> +
>> + if (state->bucket > udptable->mask) {
>
> This test looks unnecessary. The for-loop below should take care of this case?

We could return early in case the iterator has reached the end of the hash table. I suppose reset of the bucket should only happen when user stops, and starts a new iterator round.

>
>> + state->bucket = 0;
>
> Reset state->bucket here looks suspicious (or at least unnecessary) also. The iterator cannot restart from the beginning. or I am missing something here? This at least requires a comment if it is really needed.
>
>> + iter->offset = 0;
>> + return NULL;
>> + }
>> +
>> +again:
>> + /* New batch for the next bucket.
>> + * Iterate over the hash table to find a bucket with sockets matching
>> + * the iterator attributes, and return the first matching socket from
>> + * the bucket. The remaining matched sockets from the bucket are batched
>> + * before releasing the bucket lock. This allows BPF programs that are
>> + * called in seq_show to acquire the bucket lock if needed.
>> + */
>> + iter->cur_sk = 0;
>> + iter->end_sk = 0;
>> + iter->st_bucket_done = false;
>> + first_sk = NULL;
>> + batch_sks = 0;
>> + offset = iter->offset;
>> +
>> + for (; state->bucket <= udptable->mask; state->bucket++) {
>> + struct udp_hslot *hslot2 = &udptable->hash2[state->bucket];
>> +
>> + if (hlist_empty(&hslot2->head)) {
>> + offset = 0;
>> + continue;
>> + }
>> + new_offset = offset;
>> +
>> + spin_lock_bh(&hslot2->lock);
>> + udp_portaddr_for_each_entry(sk, &hslot2->head) {
>> + if (seq_sk_match(seq, sk)) {
>> + /* Resume from the last iterated socket at the
>> + * offset in the bucket before iterator was stopped.
>> + */
>> + if (offset) {
>> + --offset;
>> + continue;
>> + }
>> + if (!first_sk)
>> + first_sk = sk;
>> + if (iter->end_sk < iter->max_sk) {
>> + sock_hold(sk);
>> + iter->batch[iter->end_sk++] = sk;
>> + }
>> + batch_sks++;
>> + new_offset++;
>> + }
>> + }
>> + spin_unlock_bh(&hslot2->lock);
>> +
>> + if (first_sk)
>> + break;
>> +
>> + /* Reset the current bucket's offset before moving to the next bucket. */
>> + offset = 0;
>> + }
>> +
>> + /* All done: no batch made. */
>> + if (!first_sk)
>
> Testing !iter->end_sk should be the same?

Sure, that could work too.

>
>> + goto ret;
>> +
>> + if (iter->end_sk == batch_sks) {
>> + /* Batching is done for the current bucket; return the first
>> + * socket to be iterated from the batch.
>> + */
>> + iter->st_bucket_done = true;
>> + goto ret;
>> + }
>> + if (!resized && !bpf_iter_udp_realloc_batch(iter, batch_sks * 3 / 2)) {
>> + resized = true;
>> + /* Go back to the previous bucket to resize its batch. */
>> + state->bucket--;
>> + goto again;
>> + }
>> +ret:
>> + iter->offset = new_offset;
>
> hmm... updating iter->offset looks not right and,
> does it need a new_offset?
>

This is a leftover from earlier versions. :( Sorry, I didn't do my due diligence here.

> afaict, either
>
> a) it can resume at the old bucket. In this case, the iter->offset should not change.
>
> or
>
> b) it moved to the next bucket and iter->offset should be 0.

Yes, that's the behavior I had in mind as well.

>
>> + return first_sk;
>
> &iter->batch[0] is the first_sk. 'first_sk' variable is not needed then.

It's possible that we didn't find any socket to return, or resized batch didn't go through. So we can't always rely on iter->batch[0]. As an alternative, we could return early when a socket is found. Anyway either option seems fine.