Oct 02, 2019

cgroup-v2 resource limits for apps with systemd scopes and slices

Something like 8y ago, back in 2011, it was ok to manage cgroup-imposed resource limits in parallel to systemd (e.g. via libcgroup or custom tools), but over years cgroups moved increasingly towards "unified hierarchy" (for many good reasons, outlined in kernel docs), joining controllers at first, and then creating new unified cgroup-v2 interface for it.

Latter no longer allows to separate systemd process-tracking tree from ones for controlling specific resources, so you can't put restrictions on pids without either pulling them from hierarchy that systemd uses (making it loose track of them - bad idea) or changing cgroup configuration in parallel to systemd, potentially stepping on its toes (also bad idea).

So with systemd running in unified cgroup-v2 hierarchy mode (can be compiled-in default, esp. in the future, or enabled via systemd.unified_cgroup_hierarchy on cmdline), there are two supported options to manage custom resource limits:

  • By using systemd, via its slices, scopes and services.
  • By taking over control of some cgroup within systemd hierarchy via Delegate= in units.

Delegation necessitates managing pids within that subtree outside of systemd entirely, while first one is simplier of the two, where instead of libcgroup, cgmanager or cgconf config file, you'd define all these limits in systemd .slice unit-file hierarchy like this:

# apps.slice
[Slice]

CPUWeight=30
IOWeight=30

MemoryHigh=5G
MemoryMax=8G
MemorySwapMax=1G

# apps-browser.slice
[Slice]
CPUWeight=30
IOWeight=30
MemoryHigh=3G

# apps-misc.slice
[Slice]
MemoryHigh=700M
MemoryMax=1500M

These can reside under whatever pre-defined slices (see systemctl status for full tree), including "systemd --user" slices, where users can set these up as well.

Running arbitrary desktop app under such slices can be done as a .service with all Type=/ExecStart=/ExecStop= complications or just .scope as a bunch of arbitrary unmanaged processes, using systemd-run:

% systemd-run -q --user --scope \
  --unit chrome --slice apps-browser -- chrominum

Scope will inherit all limits from specified slices, as composed into hierarchy (with the usual hyphen-to-slash translation in unit names), and auto-start/stop the scope (when all pids there exit) and all slices required.

So no extra scripts for mucking about in /sys/fs/cgroup are needed anymore, whole subtree is visible and inspectable via systemd tools (e.g. systemctl status apps.slice, systemd-cgls, systemd-cgtop and such), and can be adjusted on the fly via e.g. systemctl set-property apps-misc.slice CPUWeight=30.

My old cgroup-tools provided few extra things for checking cgroup contents from scripts easily (cgls), queueing apps from shell via cgroups and such (cgwait, "cgrc -q"), which systemctl and systemd-run don't provide, but easy to implement on top as:

% cg=$(systemctl -q --user show -p ControlGroup --value -- apps-browser.scope)
% procs=$( [ -z "$cg" ] ||
    find /sys/fs/cgroup"$cg" -name cgroup.procs -exec cat '{}' + 2>/dev/null )

Ended up wrapping long systemd-run commands along with these into cgrc wrapper shell script in spirit of old tools:

% cgrc -h
Usage: cgrc [-x] { -l | -c | -q [opts] } { -u unit | slice }
       cgrc [-x] [-q [-i N] [-t N]] [-u unit] slice -- cmd [args...]

Run command via 'systemd-run --scope'
  within specified slice, or inspect slice/scope.
Slice should be pre-defined via .slice unit and starts/stops automatically.
--system/--user mode detected from uid (--system for root, --user otherwise).

Extra options:
-u name - scope unit name, derived from command basename by default.
   Starting scope with same unit name as already running one will fail.
   With -l/-c list/check opts, restricts check to that scope instead of slice.
-q - wait for all pids in slice/scope to exit before start (if any).
   -i - delay between checks in seconds, default=7s.
   -t - timeout for -q wait (default=none), exiting with code 36 afterwards.
-l - list all pids within specified slice recursively.
-c - same as -l, but for exit code check: 35 = pids exist, 0 = empty.
-x - 'set -x' debug mode.

% cgrc apps-browser -- chrominum &
% cgrc -l apps | xargs ps u
...
% cgrc -c apps-browser || notify-send 'browser running'
% cgrc -q apps-browser ; notify-send 'browser exited'

That + systemd slice units seem to replace all old resource-management cruft with modern unified v2 tree nicely, and should probably work for another decade, at least.

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