linux-kernel - Re: [PATCH v3] selftests/cgroup: fix cpu.max tests

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Message-ID: <aHk9HHA2vMEGBiI4@slm.duckdns.org>
Date: Thu, 17 Jul 2025 08:12:44 -1000
From: Tejun Heo <tj@...nel.org>
To: Shashank Balaji <shashank.mahadasyam@...y.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
	Shinya Takumi <shinya.takumi@...y.com>
Subject: Re: [PATCH v3] selftests/cgroup: fix cpu.max tests

On Fri, Jul 04, 2025 at 08:08:41PM +0900, Shashank Balaji wrote:
> Current cpu.max tests (both the normal one and the nested one) are broken.
> 
> They setup cpu.max with 1000 us quota and the default period (100,000 us).
> A cpu hog is run for a duration of 1s as per wall clock time. This corresponds
> to 10 periods, hence an expected usage of 10,000 us. We want the measured
> usage (as per cpu.stat) to be close to 10,000 us.
> 
> Previously, this approximate equality test was done by
> `!values_close(usage_usec, expected_usage_usec, 95)`: if the absolute
> difference between usage_usec and expected_usage_usec is greater than 95% of
> their sum, then we pass. And expected_usage_usec was set to 1,000,000 us.
> Mathematically, this translates to the following being true for pass:
> 
> 	|usage - expected_usage| > (usage + expected_usage)*0.95
> 
> 	If usage > expected_usage:
> 		usage - expected_usage > (usage + expected_usage)*0.95
> 		0.05*usage > 1.95*expected_usage
> 		usage > 39*expected_usage = 39s
> 
> 	If usage < expected_usage:
> 		expected_usage - usage > (usage + expected_usage)*0.95
> 		0.05*expected_usage > 1.95*usage
> 		usage < 0.0256*expected_usage = 25,600 us
> 
> Combined,
> 
> 	Pass if usage < 25,600 us or > 39 s,
> 
> which makes no sense given that all we need is for usage_usec to be close to
> 10,000 us.
> 
> Fix this by explicitly calcuating the expected usage duration based on the
> configured quota, default period, and the duration, and compare usage_usec
> and expected_usage_usec using values_close() with a 10% error margin.
> 
> Also, use snprintf to get the quota string to write to cpu.max instead of
> hardcoding the quota, ensuring a single source of truth.
> 
> Remove the check comparing user_usec and expected_usage_usec, since on running
> this test modified with printfs, it's seen that user_usec and usage_usec can
> regularly exceed the theoretical expected_usage_usec:
> 
> 	$ sudo ./test_cpu
> 	user: 10485, usage: 10485, expected: 10000
> 	ok 1 test_cpucg_max
> 	user: 11127, usage: 11127, expected: 10000
> 	ok 2 test_cpucg_max_nested
> 	$ sudo ./test_cpu
> 	user: 10286, usage: 10286, expected: 10000
> 	ok 1 test_cpucg_max
> 	user: 10404, usage: 11271, expected: 10000
> 	ok 2 test_cpucg_max_nested
> 
> Hence, a values_close() check of usage_usec and expected_usage_usec is
> sufficient.
> 
> Fixes: a79906570f9646ae17 ("cgroup: Add test_cpucg_max_nested() testcase")
> Fixes: 889ab8113ef1386c57 ("cgroup: Add test_cpucg_max() testcase")
> Acked-by: Michal Koutný <mkoutny@...e.com>
> Signed-off-by: Shashank Balaji <shashank.mahadasyam@...y.com>

Applied to cgroup/for-6.17.

Thanks.

-- 
tejun