selftests/cgroup: test pids.events notification

Add a test to test_pids.c that checks a fork rejected by pids.max
increments pids.events and generates an IN_MODIFY inotify event on that
file. The test cgroup is the one over the limit, so the notification is
expected both on a default mount and with pids_localevents.

This covers the fix in commit 1765a153d985 ("cgroup/pids: Restore
pids.events notifications in local mode"), where a rejected fork changed
pids.events without waking its pollers when pids_localevents was enabled.

Tested on x86_64 under QEMU without KVM, on kernels built from the
cgroup for-7.4 branch (eb69e955de7b) and from the same tree with that
fix reverted. With the fix, the new test passes on a default cgroup2
mount and on a pids_localevents mount. With the fix reverted, it passes
on a default mount and fails on a pids_localevents mount. No kernel
warnings were logged.

Link: https://lore.kernel.org/r/20261013064812.875787-1-sahaj123.sc@gmail.com
Signed-off-by: Sahaj Chaudhari <sahaj123.sc@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

0 files changed, 0 insertions, 0 deletions