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

[Wang Nan <wangnan0@huawei.com>]

On 2015/5/6 12:56, Alexei Starovoitov wrote:
> 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.
> 

That's great! Thanks to your response!