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Date: Tue, 19 Nov 2024 12:47:14 +0100
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
Message-ID: <20241119114714.GD2328@noisy.programming.kicks-ass.net>
References: <20241118185245.1065000-1-vadfed@meta.com>
 <20241118185245.1065000-5-vadfed@meta.com>
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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?