Re: [PATCH v2] perf metrics: Add literal for system TSC frequency

[Peter Zijlstra <peterz@infradead.org>]

On Thu, May 26, 2022 at 09:04:07PM -0700, Ian Rogers wrote:
> Such a literal is useful to calculate things like the average frequency
> [1]. The TSC frequency isn't exposed by sysfs although some experimental
> drivers look to add it [2]. This change computes the value using the
> frequency in /proc/cpuinfo which is accurate at least on Intel
> processors.
> 
> v2. Adds warnings to make clear if things have changed/broken on future
>     Intel platforms. It also adds caching and an Intel specific that a
>     value is computed.
> 
> [1] https://github.com/intel/perfmon-metrics/blob/5ad9ef7056f31075e8178b9f1fb732af183b2c8d/SKX/metrics/perf/skx_metric_perf.json#L11
> [2] https://github.com/trailofbits/tsc_freq_khz

This all seems bonghits inspired... and perf actually does expose the
tsc frequency. What do you think is in perf_event_mmap_page::time_* ?

> 
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/perf/tests/expr.c | 15 +++++++++++++
>  tools/perf/util/expr.c  | 50 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 65 insertions(+)
> 
> diff --git a/tools/perf/tests/expr.c b/tools/perf/tests/expr.c
> index 5c0032fe93ae..45afe4f24859 100644
> --- a/tools/perf/tests/expr.c
> +++ b/tools/perf/tests/expr.c
> @@ -1,8 +1,10 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include "util/debug.h"
>  #include "util/expr.h"
> +#include "util/header.h"
>  #include "util/smt.h"
>  #include "tests.h"
> +#include <math.h>
>  #include <stdlib.h>
>  #include <string.h>
>  #include <linux/zalloc.h>
> @@ -69,6 +71,11 @@ static int test__expr(struct test_suite *t __maybe_unused, int subtest __maybe_u
>  	double val, num_cpus, num_cores, num_dies, num_packages;
>  	int ret;
>  	struct expr_parse_ctx *ctx;
> +	bool is_intel = false;
> +	char buf[128];
> +
> +	if (!get_cpuid(buf, sizeof(buf)))
> +		is_intel = strstr(buf, "Intel") != NULL;
>  
>  	TEST_ASSERT_EQUAL("ids_union", test_ids_union(), 0);
>  
> @@ -175,6 +182,14 @@ static int test__expr(struct test_suite *t __maybe_unused, int subtest __maybe_u
>  	if (num_dies) // Some platforms do not have CPU die support, for example s390
>  		TEST_ASSERT_VAL("#num_dies >= #num_packages", num_dies >= num_packages);
>  
> +	if (is_intel) {
> +		double system_tsc_freq;
> +
> +		TEST_ASSERT_VAL("#system_tsc_freq", expr__parse(&system_tsc_freq, ctx,
> +								"#system_tsc_freq") == 0);
> +		TEST_ASSERT_VAL("!isnan(#system_tsc_freq)", !isnan(system_tsc_freq));
> +	}
> +
>  	/*
>  	 * Source count returns the number of events aggregating in a leader
>  	 * event including the leader. Check parsing yields an id.
> diff --git a/tools/perf/util/expr.c b/tools/perf/util/expr.c
> index 675f318ce7c1..f33aeb1e6faa 100644
> --- a/tools/perf/util/expr.c
> +++ b/tools/perf/util/expr.c
> @@ -402,6 +402,51 @@ double expr_id_data__source_count(const struct expr_id_data *data)
>  	return data->val.source_count;
>  }
>  
> +/*
> + * Derive the TSC frequency in Hz from the /proc/cpuinfo, for example:
> + * ...
> + * model name      : Intel(R) Xeon(R) Gold 6154 CPU @ 3.00GHz
> + * ...
> + * will return 3000000000.
> + */
> +static double system_tsc_freq(void)
> +{
> +	static double result;
> +	static bool computed;
> +	FILE *cpuinfo;
> +	char *line = NULL;
> +	size_t len = 0;
> +
> +	if (computed)
> +		return result;
> +
> +	computed = true;
> +	result = NAN;
> +	cpuinfo = fopen("/proc/cpuinfo", "r");
> +	if (!cpuinfo) {
> +		pr_err("Failed to read /proc/cpuinfo for TSC frequency");
> +		return NAN;
> +	}
> +	while (getline(&line, &len, cpuinfo) > 0) {
> +		if (!strncmp(line, "model name", 10)) {
> +			char *pos = strstr(line + 11, " @ ");
> +
> +			if (pos && sscanf(pos, " @ %lfGHz", &result) == 1) {
> +				result *= 1000000000;
> +				goto out;
> +			}
> +		}
> +	}
> +
> +out:
> +	if (isnan(result))
> +		pr_err("Failed to find TSC frequency in /proc/cpuinfo");
> +
> +	free(line);
> +	fclose(cpuinfo);
> +	return result;
> +}
> +
>  double expr__get_literal(const char *literal)
>  {
>  	static struct cpu_topology *topology;
> @@ -417,6 +462,11 @@ double expr__get_literal(const char *literal)
>  		goto out;
>  	}
>  
> +	if (!strcasecmp("#system_tsc_freq", literal)) {
> +		result = system_tsc_freq();
> +		goto out;
> +	}
> +
>  	/*
>  	 * Assume that topology strings are consistent, such as CPUs "0-1"
>  	 * wouldn't be listed as "0,1", and so after deduplication the number of
> -- 
> 2.36.1.124.g0e6072fb45-goog
>