Auto merge of #97925 - the8472:cgroupv1, r=joshtriplett

Add cgroupv1 support to available_parallelism

Fixes #97549

My dev machine uses cgroup v2 so I was only able to test that code path. So the v1 code path is written only based on documentation. I could use some help testing that it works on a machine with cgroups v1:

```
$ x.py build --stage 1

# quota.rs
fn main() {
    println!("{:?}", std::thread::available_parallelism());
}

# assuming stage1 is linked in rustup
$ rust +stage1 quota.rs

# spawn a new cgroup scope for the current user
$ sudo systemd-run -p CPUQuota="300%" --uid=$(id -u) -tdS

# should print Ok(3)
$ ./quota
```

If it doesn't work as expected an strace, the contents of `/proc/self/cgroups` and the structure of `/sys/fs/cgroups` would help.

1 files changed, 6 insertions, 1 deletions

diff --git a/library/std/src/thread/mod.rs b/library/std/src/thread/mod.rs
index d28c7b58b20..44c8a50fd86 100644
--- a/library/std/src/thread/mod.rs
+++ b/library/std/src/thread/mod.rs
@@ -1577,10 +1577,15 @@ fn _assert_sync_and_send() {
 ///
 /// On Linux:
 /// - It may overcount the amount of parallelism available when limited by a
-///   process-wide affinity mask or cgroup quotas and cgroup2 fs or `sched_getaffinity()` can't be
+///   process-wide affinity mask or cgroup quotas and `sched_getaffinity()` or cgroup fs can't be
 ///   queried, e.g. due to sandboxing.
 /// - It may undercount the amount of parallelism if the current thread's affinity mask
 ///   does not reflect the process' cpuset, e.g. due to pinned threads.
+/// - If the process is in a cgroup v1 cpu controller, this may need to
+///   scan mountpoints to find the corresponding cgroup v1 controller,
+///   which may take time on systems with large numbers of mountpoints.
+///   (This does not apply to cgroup v2, or to processes not in a
+///   cgroup.)
 ///
 /// On all targets:
 /// - It may overcount the amount of parallelism available when running in a VM