Re: [PATCH bpf-next v7 4/4] selftests/bpf: add usage example for cpu cycles kfuncs

[Vadim Fedorenko <vadim.fedorenko@linux.dev>]

On 19/11/2024 03:47, Peter Zijlstra wrote:
> On Mon, Nov 18, 2024 at 10:52:45AM -0800, Vadim Fedorenko wrote:
> 
>> +int bpf_cpu_cycles(void)
>> +{
>> +	struct bpf_pidns_info pidns;
>> +	__u64 start;
>> +
>> +	start = bpf_get_cpu_cycles();
>> +	bpf_get_ns_current_pid_tgid(0, 0, &pidns, sizeof(struct bpf_pidns_info));
>> +	cycles = bpf_get_cpu_cycles() - start;
>> +	ns = bpf_cpu_cycles_to_ns(cycles);
>> +	return 0;
>> +}
> 
> Oh, the intent is to use that cycles_to_ns() on deltas. That wasn't at
> all clear.

Yep, that's the main use case, it was discussed in the previous
versions of the patchset.

> 
> Anyway, the above has more problems than just bad naming. TSC is
> constant and not affected by DVFS, so depending on the DVFS state of
> things your function will return wildly different readings.

Why should I care about DVFS? The use case is to measure the time spent
in some code. If we replace it with bpf_ktime_get_ns(), it will also be
affected by DVFS, and it's fine. We will be able to see the difference
for different DVFS states.

> Why do you think you need this?