Re: [RFC PATCH 0/5] Make eBPF programs output data to perf event

[Alexei Starovoitov]

On 7/1/15 8:38 PM, He Kuang wrote:
> On 2015/7/2 10:48, Alexei Starovoitov wrote:
> > On 7/1/15 4:58 AM, Peter Zijlstra wrote:
> > > But why create a separate trace buffer, it should go into the regular
> > > perf buffer.
> >
> > +1
> >
> > I think
> > +static char __percpu *perf_extra_trace_buf[PERF_NR_CONTEXTS];
> > is redundant.
> > It adds quite a bit of unnecessary complexity to the whole patch set.
> >
> > Also the call to bpf_output_sample() is not effective unless program
> > returns 1. It's a confusing user interface.
> >
> > Also you cannot ever do:
> >       BPF_FUNC_probe_read,
> > +    BPF_FUNC_output_sample,
> >       BPF_FUNC_ktime_get_ns,
> > new functions must be added to the end.
> >
> > Why not just do:
> > perf_trace_buf_prepare() + perf_trace_buf_submit() from the helper?
> > No changes to current code.
> > No need to call __get_data_size() and other overhead.
> > The helper can be called multiple times from the same program.
> > imo much cleaner.
>
> Invoke perf_trace_buf_submit() will generate a second perf
> event (header->type = PERF_RECORD_SAMPLE) entry which is
> different from the event entry outputed by the orignial
> kprobe. So the final result of the example in 00/00 patch may
> like this:
>
>    sample entry 1(from bpf_prog):
>      comm timestamp1 generic_perform_write pmu_value=0x1234
>    sample entry 2(from original kprobe):
>      comm timestamp2 generic_perform_write: (ffffffff81140b60)
> Compared with current implementation:
>    combined sample entry:
>      comm timestamp generic_perform_write: (ffffffff81140b60)
> pmu_value=0x1234
>
> The former two entries may be discontinuous as there are multiple
> threads and kprobes to be recorded, and there's a chance that one
> entry is missed but the other is recorded. What we need is the
> pmu_value read when 'generic_perform_write' enters, the two
> entries result is not intuitive enough and userspace tools have
> to do the work to find and combine those two sample entries to
> get the result.

Just change your example to return 0 and user space will see
one sample.

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