Re: [PATCH net-next 1/4] bpf: add helper bpf_perf_read_counter_time for perf event array map

[Yonghong Song <yhs@fb.com>]

On 9/1/17 1:50 PM, Peter Zijlstra wrote:
> On Fri, Sep 01, 2017 at 01:29:17PM -0700, Alexei Starovoitov wrote:
> 
>>> +BPF_CALL_4(bpf_perf_read_counter_time, struct bpf_map *, map, u64, flags,
>>> +	struct bpf_perf_counter_time *, buf, u32, size)
>>> +{
>>> +	struct perf_event *pe;
>>> +	u64 now;
>>> +	int err;
>>> +
>>> +	if (unlikely(size != sizeof(struct bpf_perf_counter_time)))
>>> +		return -EINVAL;
>>> +	err = get_map_perf_counter(map, flags, &buf->counter, &pe);
>>> +	if (err)
>>> +		return err;
>>> +
>>> +	calc_timer_values(pe, &now, &buf->time.enabled, &buf->time.running);
>>> +	return 0;
>>> +}
>>
>> Peter,
>> I believe we're doing it correctly above.
>> It's a copy paste of the same logic as in total_time_enabled/running.
>> We cannot expose total_time_enabled/running to bpf, since they are
>> different counters. The above two are specific to bpf usage.
>> See commit log.
> 
> No, the patch is atrocious and the usage is wrong.
> 
> Exporting a function called 'calc_timer_values' is a horrible violation
> of the namespace.
> 
> And its wrong because it should be done in conjunction with
> perf_event_read_local(). You cannot afterwards call this because you
> don't know if the event was active when you read it and you don't have
> temporal guarantees; that is, reading these timestamps long after or
> before the read is wrong, and this interface allows it.

Thanks for explanation. Will push the read/calculate time 
enabled/running inside the perf_event_read_local then.

> 
> So no, sorry this is just fail.
>