Incorrect BPF stats accounting for fentry on arm64

[Jose Fernandez]

On 24/04/11 11:09AM, Ivan Babrou wrote:
> Hello,
> 
> We're seeing incorrect data for bpf runtime stats on arm64. Here's an example:
> 
> $ sudo bpftool prog show id 693110
> 693110: tracing  name __tcp_retransmit_skb  tag e37be2fbe8be4726  gpl
> run_time_ns 2493581964213176 run_cnt 1133532 recursion_misses 1
>     loaded_at 2024-04-10T22:33:09+0000  uid 62727
>     xlated 312B  jited 344B  memlock 4096B  map_ids 8550445,8550441
>     btf_id 8726522
>     pids prometheus-ebpf(2224907)
> 
> According to bpftool, this program reported 66555800ns of runtime at
> one point and then it jumped to 2493581675247416ns just 53s later when
> we looked at it again. This is happening only on arm64 nodes in our
> fleet on both v6.1.82 and v6.6.25.
> 
> We have two services that are involved:
> 
> * ebpf_exporter attaches bpf programs to the kernel and exports
> prometheus metrics and opentelementry traces driven by its probes
> * bpf_stats_exporter runs bpftool every 53s to capture bpf runtime metrics
> 
> The problematic fentry is attached to __tcp_retransmit_skb, but an
> identical one is also attached to tcp_send_loss_probe, which does not
> exhibit the same issue:
> 
> SEC("fentry/__tcp_retransmit_skb")
> int BPF_PROG(__tcp_retransmit_skb, struct sock *sk)
> {
>   return handle_sk((struct pt_regs *) ctx, sk, sk_kind_tcp_retransmit_skb);
> }
> 
> SEC("fentry/tcp_send_loss_probe")
> int BPF_PROG(tcp_send_loss_probe, struct sock *sk)
> {
>   return handle_sk((struct pt_regs *) ctx, sk, sk_kind_tcp_send_loss_probe);
> }
> 
> In handle_sk we do a map lookup and an optional ringbuf push. There is
> no sleeping (I don't think it's even allowed on v6.1). It's
> interesting that it only happens for the retransmit, but not for the
> loss probe.
> 
> The issue manifests some time after we restart ebpf_exporter and
> reattach the probes. It doesn't happen immediately, as we need to
> capture metrics 53s apart to produce a visible spike in metrics.
> 
> There is no corresponding spike in execution count, only in execution time.
> 
> It doesn't happen deterministically. Some ebpf_exporter restarts show
> it, some don't.
> 
> It doesn't keep happening after ebpf_exporter restart. It happens once
> and that's it.
> 
> Maybe recursion_misses plays a role here? We see none for
> tcp_send_loss_probe. We do see some for inet_sk_error_report
> tracepoint, but it doesn't spike like __tcp_retransmit_skb does.
> 
> The biggest smoking gun is that it only happens on arm64.
> 
> I'm happy to try out patches to figure this one out.

Ivan, I recently submitted a patch that improves how the bpf runtime stats are
calculated. I'm not sure if it will fix your issue, but it would be useful to
see if removing the intrumentation time from the runtime calculation helps.

https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=ce09cbdd9888

You can also use bpftop to chart bpf stats in a time series graph. Visualizing 
the stats that way may help surface more patterns about the issue.

https://github.com/Netflix/bpftop