Re: [RFC PATCH 00/22] perf tools: introduce 'perf bpf' command to load eBPF programs.

[Alexei Starovoitov]

On 5/5/15 9:46 PM, Wang Nan wrote:
> Hi Alexei Starovoitov,
>
> Have you ever read this mail?

please don't top post.

> > > all makes sense and your use case fits quite well into existing
> > > bpf+kprobe model. I'm not sure why you're calling a 'problem'.
> > > A problem of how to display that call stack from perf?
> > > I would say it fits better as a sample than a trace.
> > > If you dump it as a trace, it won't easy to decipher, whereas if you
> > > treat it a sampling event, perf record/report facility will pick it up and display nicely. Meaning that one sample == lock_page/unlock_page
> > > latency > N. Then existing sample_callchain flag should work.
> >
> > Quite well. Do we have an eBPF function like
> >
> > static int (*bpf_perf_sample)(const char *fmt, int fmt_size, ...) = BPF_FUNC_perf_sample
> >
> > so we can use it in the program probed in the body of __unlock_page() like that:
> >
> >   ...
> >   if (latency > 0.5s)
> >      bpf_perf_sample("page=%p, latency=%d", sizeof(...), page, latency);

No need for extra helper. There is already return value from
the program for this purpose.
From kernel/trace/bpf_trace.c:
 * Return: BPF programs always return an integer which is interpreted by
 * kprobe handler as:
 * 0 - return from kprobe (event is filtered out)
 * 1 - store kprobe event into ring buffer

in your case the program attached to unlock_page() can return 1
when it needs to store this event into ring buffer, so that perf can
process it. If I'm not mistaken, the sample_callchain flag cannot be
applied to kprobe events, but that's a general program (not
related to bpf) and can be addressed as such.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/