From: Peter Zijlstra <peterz@infradead.org>
To: Vadim Fedorenko <vadfed@meta.com>
Cc: Borislav Petkov <bp@alien8.de>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
Eduard Zingerman <eddyz87@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
Yonghong Song <yonghong.song@linux.dev>,
Vadim Fedorenko <vadim.fedorenko@linux.dev>,
Mykola Lysenko <mykolal@fb.com>,
x86@kernel.org, bpf@vger.kernel.org,
Martin KaFai Lau <martin.lau@linux.dev>
Subject: Re: [PATCH bpf-next v7 4/4] selftests/bpf: add usage example for cpu cycles kfuncs
Date: Tue, 19 Nov 2024 12:47:14 +0100 [thread overview]
Message-ID: <20241119114714.GD2328@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20241118185245.1065000-5-vadfed@meta.com>
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.
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 do you think you need this?
next prev parent reply other threads:[~2024-11-19 11:47 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-11-18 18:52 [PATCH bpf-next v7 0/4] bpf: add cpu cycles kfuncss Vadim Fedorenko
2024-11-18 18:52 ` [PATCH bpf-next v7 1/4] bpf: add bpf_get_cpu_cycles kfunc Vadim Fedorenko
2024-11-19 11:18 ` Peter Zijlstra
2024-11-19 14:29 ` Vadim Fedorenko
2024-11-19 16:17 ` Peter Zijlstra
2024-11-19 18:03 ` Vadim Fedorenko
2024-11-19 19:16 ` Andrii Nakryiko
2024-11-19 19:27 ` Vadim Fedorenko
2024-11-18 18:52 ` [PATCH bpf-next v7 2/4] bpf: add bpf_cpu_cycles_to_ns helper Vadim Fedorenko
2024-11-19 11:28 ` Peter Zijlstra
2024-11-19 14:38 ` Vadim Fedorenko
2024-11-20 8:49 ` Peter Zijlstra
2024-11-20 13:39 ` Vadim Fedorenko
2024-11-18 18:52 ` [PATCH bpf-next v7 3/4] selftests/bpf: add selftest to check rdtsc jit Vadim Fedorenko
2024-11-18 18:52 ` [PATCH bpf-next v7 4/4] selftests/bpf: add usage example for cpu cycles kfuncs Vadim Fedorenko
2024-11-19 11:47 ` Peter Zijlstra [this message]
2024-11-19 14:45 ` Vadim Fedorenko
2024-11-20 8:51 ` Peter Zijlstra
2024-11-20 17:19 ` Vadim Fedorenko
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