On 7/20/26 11:01 AM, Michal Koutný wrote:
On Fri, Jul 17, 2026 at 03:18:14PM -0400, Waiman Long <[email protected]>
wrote:
With commit 98149f542530 ("selftests/cgroup: Add test for cpuset affinity
on controller disable"), sashiko [1] had report 3 different issues with
the new test_cpuset_affinity_on_controller_disable() test.
1) `cpu_set_equal` iterates over mask bytes instead of bits, ignoring
CPUs >= 8.
Inline comment
2) Thread synchronization logic allows the main thread to read
uninitialized stack memory, causing test flakiness.
Hm, I cannot see it (alhtough I don't see it through), what was the
stack memory?
(test_phase is static, then re-initalized)
I believe it means the followings:
cpu_set_t affinity_a_before, affinity_a_after;
cpu_set_t affinity_b_before, affinity_b_after;
These variables are supposed to be set by child_a and child_b, but it is
possible that child_b runs first, set ready_phase to
AFFINITY_THREADS_READY before child_a run and set affinity_a_before
which can be any value depending on its previous state of the stack. So
the subsequent cpu_set_equal(&affinity_a_before, 0x3) call can pass or
fail. That is what I believe the problem is.
3) Test fails instead of skipping gracefully on uniprocessor systems
or when CPU 1 is unavailable.
Interesting catch.
Fix the reported issues by:
1) Iterates over the bit size of the mask.
2) Test the new ready flag for each thread to end the wait
on the condoitional variable and eliminate the now unneeded
AFFINITY_THREAD_A_READY and AFFINITY_THREADS_READY test phases.
But the symmetric synchronization with counter is easier to reason
about.
3) Return KSFT_SKIP on "cpuset.cpus" setting failure.
It'd be better to have same style with test_cpuset_prs.sh, i.e. a guard
at the beginning requesting a minimal number of CPUs. Next time...
Yes, that can be another alternative. It is just that the current fix is
easier.
@@ -251,7 +251,7 @@ static int cpu_set_equal(cpu_set_t *dst, unsigned long mask)
CPU_ZERO(&expected);
assert(sizeof(mask) < CPU_SETSIZE);
- for (int cpu = 0; cpu < sizeof(mask); ++cpu)
+ for (int cpu = 0; cpu < sizeof(mask) * 8; ++cpu)
if ((1UL << cpu) & mask)
CPU_SET(cpu, &expected);
Oh, that was my braino in how masks are stored.
Thanks for correcting me!
It should also extend the assert accordingly:
assert(sizeof(mask) * 8 < CPU_SETSIZE);
I don't think we need an assertion like that as CPU_SETSIZE will always
be a multiple of a long bit size . Perhaps we could have something like
int max = min(sizeof(mask) * 8, CPU_SETSIZE);
for (...; cpu < max; ...)
Maybe next time when we need to update test_cpuset.c.
Cheers,
Longman
