linux-kernel - Re: [PATCH 0/2] selftests/cgroup: better bound for cpu.max tests

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Message-ID: <aGXfvfKOjWlH3d0q@JPC00244420>
Date: Thu, 3 Jul 2025 10:41:17 +0900
From: Shashank Balaji <shashank.mahadasyam@...y.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
	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 0/2] selftests/cgroup: better bound for cpu.max tests

Hi Michal, 

Thanks for the reply!

On Wed, Jul 02, 2025 at 02:34:29PM +0200, Michal Koutný wrote:
> Hello Shashank.
> 
> On Tue, Jul 01, 2025 at 11:13:54PM +0900, Shashank Balaji <shashank.mahadasyam@...y.com> wrote:
> > cpu.max selftests (both the normal one and the nested one) test the
> > working of throttling by setting up cpu.max, running a cpu hog process
> > for a specified duration, and comparing usage_usec as reported by
> > cpu.stat with the duration of the cpu hog: they should be far enough.
> > 
> > Currently, this is done by using values_close, which has two problems:
> > 
> > 1. Semantic: values_close is used with an error percentage of 95%, which
> >    one will not expect on seeing "values close". The intent it's
> > actually going for is "values far".
> > 
> > 2. Accuracy: the tests can pass even if usage_usec is upto around double
> >    the expected amount. That's too high of a margin for usage_usec.
> > 
> > Overall, this patchset improves the readability and accuracy of the
> > cpu.max tests.
> > 
> > Signed-off-by: Shashank Balaji <shashank.mahadasyam@...y.com>
> 
> I think you're getting at an actual bug in the test definition. 
> 
> I think that the test_cpucg_max should either run hog_cpus_timed with
> CPU_HOG_CLOCK_PROCESS instead of CPU_HOG_CLOCK_WALL to make sense or the
> expected_usage_usec should be defined with the configured quota in mind
> (i.e. 1/100).  (The latter seems to make the test more natural.)

Going with the more natural way of sticking to CPU_HOG_CLOCK_WALL, the
second patch does calculate expected_usage_usec based on the configured
quota, as the code comment explains. So I'm guessesing we're on the same page
about this?

> With such defined metrics, the asserted expression could be
> 	values_close(usage_usec, expected_usage_usec, 10)
> based on your numbers, error is around 20% so our helper's argument is
> roughly half of that. (I'd be fine even with err=20 to prevent some
> false positives.)
> 
> I think those changes could even be in one patch but I leave that up to
> you. My comment to your 2nd patch is that I'd like to stick to relative
> errors and keep positive values_close() predicate that's used in other
> selftests too. (But those 95% in the current code are clumsy given two
> different qualities are compared.)

Do you mean something like,

	if (values_close(usage_usec, expected_usage_usec, 10))
			goto cleanup;

using the positive values_close() predicate. If so, I'm not sure I
understand because if usage_usec and expected_usage_usec _are_ close,
then we want the test to pass! We should be using the negative
predicate.

And sure, I'll send v2 as a single patch.

Thanks

Shashank