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?
