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

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Message-ID: <55499F1B.1020302@plumgrid.com>
Date:	Tue, 05 May 2015 21:56:59 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Wang Nan <wangnan0@...wei.com>
CC:	linux-kernel@...r.kernel.org, pi3orama@....com,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [RFC PATCH 00/22] perf tools: introduce 'perf bpf' command to
 load eBPF programs.

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.

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