Re: [PATCH bpf-next] bpf: Add bpf_read_raw_record() helper

[Song Liu <songliubraving@fb.com>]

> On Aug 26, 2022, at 2:12 PM, Namhyung Kim <namhyung@kernel.org> wrote:
> 
> On Fri, Aug 26, 2022 at 1:59 PM Song Liu <songliubraving@fb.com> wrote:
>> 
>> 
>> 
>>> On Aug 26, 2022, at 12:30 PM, Namhyung Kim <namhyung@kernel.org> wrote:
>>> 
>>> On Fri, Aug 26, 2022 at 11:45 AM Song Liu <songliubraving@fb.com> wrote:
>>> 
>>>>> And actually, we can just read ctx->data and get the raw record,
>>>>> right..?
>>>> 
>>>> Played with this for a little bit. ctx->data appears to be not
>>>> reliable sometimes. I guess (not 100% sure) this is because we
>>>> call bpf program before event->orig_overflow_handler. We can
>>>> probably add a flag to specify we want to call orig_overflow_handler
>>>> first.
>>> 
>>> I'm not sure.  The sample_data should be provided by the caller
>>> of perf_event_overflow.  So I guess the bpf program should see
>>> a valid ctx->data.
>> 
>> Let's dig into this. Maybe we need some small changes in
>> pe_prog_convert_ctx_access.
> 
> Sure, can you explain the problem in detail and share your program?

I push the code to 

 https://git.kernel.org/pub/scm/linux/kernel/git/song/linux.git/log/?h=test-perf-event

The code is in tools/bpf/perf-test/. 

The problem is we cannot get reliable print of data->cpu_entry in 
/sys/kernel/tracing/trace. 

> 
>> 
>>> Also I want to control calling the orig_overflow_handler based
>>> on the return value of the BPF program.  So calling the orig
>>> handler before BPF won't work for me. :)
>> 
>> Interesting. Could you share more information about the use case?
> 
> Well.. it's nothing new.  The bpf_overflow_handler calls the
> orig_overflow_handler (which writes the sample to the buffer)
> only if the BPF returns non zero.  Then I can drop unnecessary
> samples based on the sample data by returning 0.
> 
> The possible use cases are
> 1. when you want to sample from specific code ranges only
> 2. when hardware sets specific bits in raw data

I like this idea. We already using BPF in counting perf_event. 
Now it is time to use it in sampling perf_event. :)

Thanks,
Song