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

[Song Liu]

> On Aug 26, 2022, at 11:25 PM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> 
> On Fri, Aug 26, 2022 at 2:26 PM Song Liu <songliubraving@xxxxxx> wrote:
>> 
>> 
>> 
>>> On Aug 26, 2022, at 2:12 PM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>>> 
>>> On Fri, Aug 26, 2022 at 1:59 PM Song Liu <songliubraving@xxxxxx> wrote:
>>>> 
>>>> 
>>>> 
>>>>> On Aug 26, 2022, at 12:30 PM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>>>>> 
>>>>> On Fri, Aug 26, 2022 at 11:45 AM Song Liu <songliubraving@xxxxxx> 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.
> 
> Ah, right.  I've realized that the sample data is passed before full
> initialized.  Please see perf_sample_data_init().  The other members
> are initialized right before written to the ring buffer in the
> orig_overflow_handler (__perf_event_output).
> 
> That explains why pe_prog_convert_ctx_access() handles
> data and period specially.  We need to handle it first.

Thanks for confirming this. I guess we will need a helper (or kfunc) 
for the raw data. 

Shall we make it more generic that we can get other PERF_SAMPLE_*? 

Thanks,
Song