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

From: Song Liu
Date: Mon Aug 29 2022 - 03:21:06 EST




> 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