linux-kernel - Re: [PATCH v3 2/2] cgroup/rstat: Selftests for niced CPU statistics

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Message-ID: <xmayvi6p6brlx3whqcgv2wzniggrfdfqq7wnl3ojzme5kvfwpy@65ijmy7s2tye>
Date: Thu, 26 Sep 2024 20:10:13 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Joshua Hahn <joshua.hahnjy@...il.com>
Cc: tj@...nel.org, cgroups@...r.kernel.org, hannes@...xchg.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, lizefan.x@...edance.com, 
	shuah@...nel.org
Subject: Re: [PATCH v3 2/2] cgroup/rstat: Selftests for niced CPU statistics

On Mon, Sep 23, 2024 at 07:20:06AM GMT, Joshua Hahn <joshua.hahnjy@...il.com> wrote:
> +/*
> + * Creates a nice process that consumes CPU and checks that the elapsed
> + * usertime in the cgroup is close to the expected time.
> + */
> +static int test_cpucg_nice(const char *root)
> +{
> +	int ret = KSFT_FAIL;
> +	int status;
> +	long user_usec, nice_usec;
> +	long usage_seconds = 2;
> +	long expected_nice_usec = usage_seconds * USEC_PER_SEC;
> +	char *cpucg;
> +	pid_t pid;
> +
> +	cpucg = cg_name(root, "cpucg_test");
> +	if (!cpucg)
> +		goto cleanup;
> +
> +	if (cg_create(cpucg))
> +		goto cleanup;
> +
> +	user_usec = cg_read_key_long(cpucg, "cpu.stat", "user_usec");
> +	nice_usec = cg_read_key_long(cpucg, "cpu.stat", "nice_usec");
> +	if (user_usec != 0 || nice_usec != 0)
> +		goto cleanup;

Can you please distinguish a check between non-zero nice_usec and
non-existent nice_usec (KSFT_FAIL vs KSFT_SKIP)? So that the selftest is
usable on older kernels too.

> +
> +	/*
> +	 * We fork here to create a new process that can be niced without
> +	 * polluting the nice value of other selftests
> +	 */
> +	pid = fork();
> +	if (pid < 0) {
> +		goto cleanup;
> +	} else if (pid == 0) {
> +		struct cpu_hog_func_param param = {
> +			.nprocs = 1,
> +			.ts = {
> +				.tv_sec = usage_seconds,
> +				.tv_nsec = 0,
> +			},
> +			.clock_type = CPU_HOG_CLOCK_PROCESS,
> +		};
> +
> +		/* Try to keep niced CPU usage as constrained to hog_cpu as possible */
> +		nice(1);
> +		cg_run(cpucg, hog_cpus_timed, (void *)&param);

Notice that cg_run() does fork itself internally.
So you can call hog_cpus_timed(cpucg, (void *)&param) directly, no
need for the fork with cg_run(). (Alternatively substitute fork in this
test with the fork in cg_run() but with extension of cpu_hog_func_params
with the nice value.)


Thanks,
Michal

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