Re: [PATCH v9 tip 8/9] samples: bpf: IO latency analysis (iosnoop/heatmap)

Ingo Molnar
2015-03-23T07:40:37Z

* Alexei Starovoitov <ast@plumgrid.com> wrote:

> BPF C program attaches to blk_mq_start_request/blk_update_request kprobe events
> to calculate IO latency.

...

> +/* kprobe is NOT a stable ABI
> + * This bpf+kprobe example can stop working any time.
> + */
> +SEC("kprobe/blk_mq_start_request")
> +int bpf_prog1(struct pt_regs *ctx)
> +{
> + long rq = ctx->di;
> + u64 val = bpf_ktime_get_ns();
> +
> + bpf_map_update_elem(&my_map, &rq, &val, BPF_ANY);
> + return 0;
> +}

So just to make sure the original BPF instrumentation model is still
upheld: no matter in what way the kernel changes, neither the kprobe,
nor the BPF program can ever crash or corrupt the kernel, assuming the
kprobes, perf and BPF subsystem has no bugs, correct?

So 'stops working' here means that the instrumentation data might not
be reliable if kernel internal interfaces change - but it won't ever
make the kernel unreliable in any fashion. Right?

Thanks,

Ingo