Messages in this thread | | | Date | Thu, 02 Jul 2015 11:37:51 -0700 | From | Alexei Starovoitov <> | Subject | Re: [RFC PATCH 0/5] Make eBPF programs output data to perf event |
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On 7/2/15 2:24 AM, Wangnan (F) wrote: > Yes, by using perf_trace_buf_prepare() + perf_trace_buf_submit() in > helper function and let bpf program always returns 0 we can make data > collected by BPF programs output into samples, if following problems > are solved: > > 1. In bpf program there's no way to get 'struct perf_event' or 'struct > ftrace_event_call'. We have to deduce them through pt_regs: > > pt_regs -> ip -> kprobe -> struct trace_kprobe -> struct > ftrace_event_call -> hlist_entry -> struct perf_event
yeah, going through hash table via get_kprobe() is not pretty. How about using this_cpu_write(current_perf_event, ...) and using it from the helper? bpf progs are non-preemptable and non-reentrable. Also I think this helper would be more flexible if we can allow passing sample_type into it. Ideally from the program one could do something like: bpf_event_output(buf, sizeof(buf), PERF_SAMPLE_CALLCHAIN); which will prepare a sample with raw buf and callstack. This way program can decide when and how send events to user space.
> 2. Even if we finally get 'struct perf_event', I'm not sure whether > user really concern on it. If we really concern on all information > output through perf_trace_buf_submit() like callstack and > register, why not make bpf program return non-zero instead? But then > we have to consider how to connect two samples together.
see my suggestion above. when sample_type was hard coded during event creation it's a useful case on its own, but if we can make program to provide this type dynamically, it will open whole new set of possibilities.
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