Re: [PATCH bpf-next v8 0/4] bpf: add cpu cycles kfuncss

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 20, 2024 at 04:08:10PM -0800, Vadim Fedorenko wrote:
> This patchset adds 2 kfuncs to provide a way to precisely measure the
> time spent running some code. The first patch provides a way to get cpu
> cycles counter which is used to feed CLOCK_MONOTONIC_RAW. On x86
> architecture it is effectively rdtsc_ordered() function while on other
> architectures it falls back to __arch_get_hw_counter(). The second patch
> adds a kfunc to convert cpu cycles to nanoseconds using shift/mult
> constants discovered by kernel. The main use-case for this kfunc is to
> convert deltas of timestamp counter values into nanoseconds. It is not
> supposed to get CLOCK_MONOTONIC_RAW values as offset part is skipped.
> JIT version is done for x86 for now, on other architectures it falls
> back to slightly simplified version of vdso_calc_ns.

So having now read this. I'm still left wondering why you would want to
do this.

Is this just debug stuff, for when you're doing a poor man's profile
run? If it is, why do we care about all the precision or the ns. And why
aren't you using perf?

Is it something else?

Again, what are you going to do with this information?