Re: [PATCH v2] selftests/cgroup: Adjust cpu.max quota based on HZ

[Joe Simmons-Talbott <josimmon@redhat.com>]

On Tue, Jun 23, 2026 at 01:32:08PM +0800, Tao Cui wrote:
> 
> Hi Joe,
> 
> One comment on the fallback:
> 
>   quota_usec = hz != -1 ? USEC_PER_SEC / hz : 1000;
> 
> When HZ can't be determined (no CONFIG_IKCONFIG_PROC, or zcat missing),
> the fallback to 1000 is the exact value that fails at low HZ — so this
> doesn't actually fix such kernels. A larger fallback (e.g. 10000, the
> HZ=100 equivalent) would make the tests robust regardless of whether the
> config is exposed.

Hi Tao Cui,

Thank you for your review.

I am happy to use 10000 as the fallback value.  I will address this as
well as the sashiko comments in v3.

Thanks,
Joe
> 
> 在 2026/6/23 03:43, Joe Simmons-Talbott 写道:
> > For lower HZ values a quota of 1000us is much lower than the amount
> > of microseconds per tick which makes the tests test_cpucg_max and
> > test_cpugc_max_nested fail. Use the amount of microseconds per tick
> > as the quota value.
> > 
> > Signed-off-by: Joe Simmons-Talbott <joest@redhat.com>
> > ---
> > changes since v1:
> > - Try checking /proc/config.gz to get the actual kernel HZ value and
> >   fallback to 1000 if the value cannot be determined.
> > 
> >  tools/testing/selftests/cgroup/test_cpu.c | 33 +++++++++++++++++++++--
> >  1 file changed, 31 insertions(+), 2 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/cgroup/test_cpu.c b/tools/testing/selftests/cgroup/test_cpu.c
> > index 7a40d76b9548..65e09555309f 100644
> > --- a/tools/testing/selftests/cgroup/test_cpu.c
> > +++ b/tools/testing/selftests/cgroup/test_cpu.c
> > @@ -639,6 +639,29 @@ test_cpucg_nested_weight_underprovisioned(const char *root)
> >  	return run_cpucg_nested_weight_test(root, false);
> >  }
> >  
> > +/*
> > + * Best effort attempt to get the kernel's HZ value from the config.
> > + * Return the HZ value if found otherwise return -1 to indicate failure.
> > + */
> > +static long
> > +_get_config_hz(void)
> > +{
> > +	long hz = -1;
> > +	FILE *f;
> > +	char cmd[256] = "zcat /proc/config.gz 2>/dev/null | grep '^CONFIG_HZ='";
> > +
> > +	f = popen(cmd, "r");
> > +
> > +	if (!f)
> > +		goto out;
> > +
> > +	fscanf(f, "CONFIG_HZ=%ld", &hz);
> > +
> > +out:
> > +	pclose(f);
> > +	return hz;
> > +}
> > +
> >  /*
> >   * This test creates a cgroup with some maximum value within a period, and
> >   * verifies that a process in the cgroup is not overscheduled.
> > @@ -646,7 +669,8 @@ test_cpucg_nested_weight_underprovisioned(const char *root)
> >  static int test_cpucg_max(const char *root)
> >  {
> >  	int ret = KSFT_FAIL;
> > -	long quota_usec = 1000;
> > +	long hz = _get_config_hz();
> > +	long quota_usec;
> >  	long default_period_usec = 100000; /* cpu.max's default period */
> >  	long duration_seconds = 1;
> >  
> > @@ -655,6 +679,8 @@ static int test_cpucg_max(const char *root)
> >  	char *cpucg;
> >  	char quota_buf[32];
> >  
> > +	quota_usec = hz != -1 ? USEC_PER_SEC / hz : 1000;
> > +
> >  	snprintf(quota_buf, sizeof(quota_buf), "%ld", quota_usec);
> >  
> >  	cpucg = cg_name(root, "cpucg_test");
> > @@ -710,7 +736,8 @@ static int test_cpucg_max(const char *root)
> >  static int test_cpucg_max_nested(const char *root)
> >  {
> >  	int ret = KSFT_FAIL;
> > -	long quota_usec = 1000;
> > +	long quota_usec;
> > +	long hz = _get_config_hz();
> >  	long default_period_usec = 100000; /* cpu.max's default period */
> >  	long duration_seconds = 1;
> >  
> > @@ -719,6 +746,8 @@ static int test_cpucg_max_nested(const char *root)
> >  	char *parent, *child;
> >  	char quota_buf[32];
> >  
> > +	quota_usec = hz != -1 ? USEC_PER_SEC / hz : 1000;
> > +
> >  	snprintf(quota_buf, sizeof(quota_buf), "%ld", quota_usec);
> >  
> >  	parent = cg_name(root, "cpucg_parent");
> 
>