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Use cgroup quotas for calculating available_parallelism #92697

Merged
merged 3 commits into from
Mar 4, 2022
Merged

Use cgroup quotas for calculating available_parallelism #92697

merged 3 commits into from
Mar 4, 2022

Commits on Mar 2, 2022

  1. Use cgroup quotas for calculating available_parallelism 

    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::thread::available_parallelism()); // prints Ok(3)
}
```


Caveats

* cgroup v1 is ignored
* funky mountpoints (containing spaces, newlines or control chars) for cgroupfs will not be handled correctly since that would require unescaping /proc/self/mountinfo
  The escaping behavior of procfs seems to be undocumented. systemd and docker default to `/sys/fs/cgroup` so it should be fine for most systems.
* quota will be ignored when `sched_getaffinity` doesn't work
* assumes procfs is mounted under `/proc` and cgroupfs mounted and readable somewhere in the directory tree
    @the8472
    the8472 committed Mar 2, 2022
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  2. hardcode /sys/fs/cgroup instead of doing a lookup via mountinfo 

    this avoids parsing mountinfo which can be huge on some systems and
something might be emulating cgroup fs for sandboxing reasons which means
it wouldn't show up as mountpoint

additionally the new implementation operates on a single pathbuffer, reducing allocations
    @the8472
    the8472 committed Mar 2, 2022
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  3. update available_parallelism docs since cgroups and sched_getaffinity… 

    … are now taken into account
    @the8472
    the8472 committed Mar 2, 2022
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