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

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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:🧵: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.
This commit is contained in:
bors 2022-07-23 13:33:56 +00:00
commit e55c53c57e
3 changed files with 195 additions and 46 deletions
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7
library/std/src/thread/mod.rs
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@ -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