Rollup merge of #92697 - the8472:cgroups, r=joshtriplett

Use cgroup quotas for calculating `available_parallelism`

Automated tests for this are possible but would require a bunch of assumptions. It requires root + a recent kernel, systemd and maybe docker. And even then it would need a helper binary since the test has to run in a separate process.

Limitations

* only supports cgroup v2 and assumes it's mounted under `/sys/fs/cgroup`
* procfs must be available
* the quota gets mixed into `sched_getaffinity`, so if the latter doesn't work then quota information gets ignored too

Manually tested via

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

// quota.rs
#![feature(available_parallelism)]
fn main() {
    println!("{:?}", std:🧵:available_parallelism()); // prints Ok(3)
}
```

strace:

```
sched_getaffinity(3041643, 32, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47]) = 32
openat(AT_FDCWD, "/proc/self/cgroup", O_RDONLY|O_CLOEXEC) = 3
statx(0, NULL, AT_STATX_SYNC_AS_STAT, STATX_ALL, NULL) = -1 EFAULT (Bad address)
statx(3, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0444, stx_size=0, ...}) = 0
lseek(3, 0, SEEK_CUR)                   = 0
read(3, "0::/system.slice/run-u31477.serv"..., 128) = 36
read(3, "", 92)                         = 0
close(3)                                = 0
statx(AT_FDCWD, "/sys/fs/cgroup/system.slice/run-u31477.service/cgroup.controllers", AT_STATX_SYNC_AS_STAT, STATX_ALL, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0444, stx_size=0, ...}) = 0
openat(AT_FDCWD, "/sys/fs/cgroup/system.slice/run-u31477.service/cpu.max", O_RDONLY|O_CLOEXEC) = 3
statx(3, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0644, stx_size=0, ...}) = 0
lseek(3, 0, SEEK_CUR)                   = 0
read(3, "300000 100000\n", 20)          = 14
read(3, "", 6)                          = 0
close(3)                                = 0
openat(AT_FDCWD, "/sys/fs/cgroup/system.slice/cpu.max", O_RDONLY|O_CLOEXEC) = 3
statx(3, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_BASIC_STATS|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0644, stx_size=0, ...}) = 0
lseek(3, 0, SEEK_CUR)                   = 0
read(3, "max 100000\n", 20)             = 11
read(3, "", 9)                          = 0
close(3)                                = 0
openat(AT_FDCWD, "/sys/fs/cgroup/cpu.max", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
sched_getaffinity(0, 128, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47]) = 40
```

r? ```````@joshtriplett```````
cc ```````@yoshuawuyts```````

Tracking issue and previous discussion: #74479

library/std/src/sys/unix/thread.rs

@@ -279,10 +279,15 @@ pub fn available_parallelism() -> io::Result<NonZeroUsize> {
         ))] {
             #[cfg(any(target_os = "android", target_os = "linux"))]
             {
+                let quota = cgroup2_quota().max(1);
                 let mut set: libc::cpu_set_t = unsafe { mem::zeroed() };
-                if unsafe { libc::sched_getaffinity(0, mem::size_of::<libc::cpu_set_t>(), &mut set) } == 0 {
-                    let count = unsafe { libc::CPU_COUNT(&set) };
-                    return Ok(unsafe { NonZeroUsize::new_unchecked(count as usize) });
+                unsafe {
+                    if libc::sched_getaffinity(0, mem::size_of::<libc::cpu_set_t>(), &mut set) == 0 {
+                        let count = libc::CPU_COUNT(&set) as usize;
+                        let count = count.min(quota);
+                        // SAFETY: affinity mask can't be empty and the quota gets clamped to a minimum of 1
+                        return Ok(NonZeroUsize::new_unchecked(count));
+                    }
                 }
             }
             match unsafe { libc::sysconf(libc::_SC_NPROCESSORS_ONLN) } {
@@ -368,6 +373,80 @@ pub fn available_parallelism() -> io::Result<NonZeroUsize> {
     }
 }
 
+/// Returns cgroup CPU quota in core-equivalents, rounded down, or usize::MAX if the quota cannot
+/// be determined or is not set.
+#[cfg(any(target_os = "android", target_os = "linux"))]
+fn cgroup2_quota() -> usize {
+    use crate::ffi::OsString;
+    use crate::fs::{try_exists, File};
+    use crate::io::Read;
+    use crate::os::unix::ffi::OsStringExt;
+    use crate::path::PathBuf;
+
+    let mut quota = usize::MAX;
+
+    let _: Option<()> = try {
+        let mut buf = Vec::with_capacity(128);
+        // find our place in the cgroup hierarchy
+        File::open("/proc/self/cgroup").ok()?.read_to_end(&mut buf).ok()?;
+        let cgroup_path = buf
+            .split(|&c| c == b'\n')
+            .filter_map(|line| {
+                let mut fields = line.splitn(3, |&c| c == b':');
+                // expect cgroupv2 which has an empty 2nd field
+                if fields.nth(1) != Some(b"") {
+                    return None;
+                }
+                let path = fields.last()?;
+                // skip leading slash
+                Some(path[1..].to_owned())
+            })
+            .next()?;
+        let cgroup_path = PathBuf::from(OsString::from_vec(cgroup_path));
+
+        let mut path = PathBuf::with_capacity(128);
+        let mut read_buf = String::with_capacity(20);
+
+        let cgroup_mount = "/sys/fs/cgroup";
+
+        path.push(cgroup_mount);
+        path.push(&cgroup_path);
+
+        path.push("cgroup.controllers");
+
+        // skip if we're not looking at cgroup2
+        if matches!(try_exists(&path), Err(_) | Ok(false)) {
+            return usize::MAX;
+        };
+
+        path.pop();
+
+        while path.starts_with(cgroup_mount) {
+            path.push("cpu.max");
+
+            read_buf.clear();
+
+            if File::open(&path).and_then(|mut f| f.read_to_string(&mut read_buf)).is_ok() {
+                let raw_quota = read_buf.lines().next()?;
+                let mut raw_quota = raw_quota.split(' ');
+                let limit = raw_quota.next()?;
+                let period = raw_quota.next()?;
+                match (limit.parse::<usize>(), period.parse::<usize>()) {
+                    (Ok(limit), Ok(period)) => {
+                        quota = quota.min(limit / period);
+                    }
+                    _ => {}
+                }
+            }
+
+            path.pop(); // pop filename
+            path.pop(); // pop dir
+        }
+    };
+
+    quota
+}
+
 #[cfg(all(
     not(target_os = "linux"),
     not(target_os = "freebsd"),

library/std/src/thread/mod.rs

@@ -1524,7 +1524,10 @@ fn _assert_sync_and_send() {
 ///
 /// On Linux:
 /// - It may overcount the amount of parallelism available when limited by a
-///   process-wide affinity mask, or when affected by cgroup limits.
+///   process-wide affinity mask or cgroup quotas and cgroup2 fs or `sched_getaffinity()` 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.
 ///
 /// On all targets:
 /// - It may overcount the amount of parallelism available when running in a VM