netdev - Re: [PATCH v9 bpf-next 5/9] bpf: udp: Implement batching for sockets iterator

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Date: Mon, 25 Sep 2023 16:34:06 -0700
From: Aditi Ghag <aditi.ghag@...valent.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: sdf@...gle.com,
 Martin KaFai Lau <martin.lau@...nel.org>,
 bpf@...r.kernel.org,
 Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v9 bpf-next 5/9] bpf: udp: Implement batching for sockets
 iterator



> On Sep 19, 2023, at 5:38 PM, Martin KaFai Lau <martin.lau@...ux.dev> wrote:
> 
> On 5/19/23 3:51 PM, Aditi Ghag wrote:
>> +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 udp_table *udptable;
>> +	unsigned int batch_sks = 0;
>> +	bool resized = false;
>> +	struct sock *sk;
>> +
>> +	/* The current batch is done, so advance the bucket. */
>> +	if (iter->st_bucket_done) {
>> +		state->bucket++;
>> +		iter->offset = 0;
>> +	}
>> +
>> +	udptable = udp_get_table_seq(seq, net);
>> +
>> +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;
>> +	batch_sks = 0;
>> +
>> +	for (; state->bucket <= udptable->mask; state->bucket++) {
>> +		struct udp_hslot *hslot2 = &udptable->hash2[state->bucket];
>> +
>> +		if (hlist_empty(&hslot2->head)) {
>> +			iter->offset = 0;
>> +			continue;
>> +		}
>> +
>> +		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 (iter->offset) {
>> +					--iter->offset;
> 
> Hi Aditi, I think this part has a bug.
> 
> When I run './test_progs -t bpf_iter/udp6' in a machine with some udp so_reuseport sockets, this test is never finished.
> 
> A broken case I am seeing is when the bucket has >1 sockets and bpf_seq_read() can only get one sk at a time before it calls bpf_iter_udp_seq_stop().

Just so that I understand the broken case better, are you doing something in your BPF iterator program so that "bpf_seq_read() can only get one sk at a time"? 

> 
> I did not try the change yet. However, from looking at the code where iter->offset is changed, --iter->offset here is the most likely culprit and it will make backward progress for the same bucket (state->bucket). Other places touching iter->offset look fine.
> 
> It needs a local "int offset" variable for the zero test. Could you help to take a look, add (or modify) a test and fix it?
> 
> The progs/bpf_iter_udp[46].c test can be used to reproduce. The test_udp[46] in prog_tests/bpf_iter.c needs to be changed though to ensure there is multiple sk in the same bucket. Probably a few so_reuseport sk should do.


The sock_destroy patch set had added a test with multiple so_reuseport sks in a bucket in order to exercise batching [1]. I was wondering if extending the test with an additional bucket should do it, or some more cases are required (asked for clarification above) to reproduce the issue. 


[1] https://elixir.bootlin.com/linux/v6.5/source/tools/testing/selftests/bpf/prog_tests/sock_destroy.c#L146

> 
> Thanks.
> 
>> +					continue;
>> +				}
>> +				if (iter->end_
>