How a little-known flag solves big problems in cgroup management
Control Groups (cgroups) are a cornerstone of Linux resource management, enabling fine-grained control over system resources. Among various cgroup operations such as creation, deletion, and controller configuration, task classification (i.e., placing processes into the correct cgroup) is arguably the most critical.
While creating and configuring cgroups defines the resource boundaries, assigning tasks to these cgroups enforces those limits and governs actual resource usage. In essence, task classification is the consumer-side mechanism of resource management, making it vital for effective containerization, performance tuning, and system stability.
In this article, I will briefly introduce different methods for task migration, their trade-offs, and how cgrulesengd automates this process. Special attention will be given to the ignore_rt option introduced in libcgroup 3.2[1], which is designed to handle real-time tasks more gracefully.
Why Task Classification Matters
Task classification involves migrating tasks to the appropriate cgroup based on predefined rules. For example:
- A containerized database might require dedicated cpu and memory.
- Real-time applications may need priority access to resources to meet deadlines.
Misclassification can lead to resource contention, degraded performance, or even system instability. Let’s examine common approaches to task migration.
Methods of task migration
There are several approaches for assigning tasks to their respective cgroups:
1. Bash Commands/Scripts
System administrators can manually move tasks using commands like:
echo <PID> > /sys/fs/cgroup/<controller>/<cgroup>/tasks
Pros: Simple and useful for one-off changes.
Cons: Requires PID tracking and script maintenance. Its error-prone and not scalable.
2. libcgroup APIs
Applications can programmatically migrate tasks using libcgroup APIs.
C API Usage:
#include <libcgroup.h> cgroup_move_task(pid_t pid, const char *dest_cgroup);
Python Binding Usage:
from libcgroup import Cgroup Cgroup.move_process(pid, dest_cgroup, controller)
Pros: Allows custom logic and automation.
Cons: Adds dependencies and integration complexity.
3. cgexec Tool
Launch tasks directly in a cgroup using cgexec:
# cgexec -g cpu,memory:my_cgroup ./my_app
Pros: Avoids manual PID tracking and API integration.
Cons: Requires modifying startup commands, which may not suit legacy applications.
Note: One notable exclusion here is systemd[2], which uses its own internal rules and unit files to manage cgroups. Since this article focuses on libcgroup-based handling, systemd-specific behavior is out of scope.
Introducing cgrulesengd: Automated task classification
Imagine if tasks could automatically be placed into the correct cgroup without scripts, API calls, or custom launch commands. That’s exactly what cgrulesengd, a daemon provided by libcgroup, enables.
This background service monitors process creation and migrates tasks to the appropriate cgroups based on rules defined in /etc/cgrules.conf.
How it works
- Rule Definition: Define mapping rules in
/etc/cgrules.confthat associate users, groups, or commands with target cgroups. - Daemon Execution: cgrulesengd enforces these rules either as a persistent daemon or a one-shot tool.
cgrules.conf – Example usecases
- Move all tasks created by
databaseuser, into theDBcgroup:
database * DB
- Ignore all tasks created by database user, preventing them from being migrated:
database * DB ignore
For more details on writing a rule, refer cgrules.conf(5) man page [3].
Handling Real-Time tasks: A critical caveat
Dealing with real-time (RT) tasks introduces complexity. This article does not dive into RT tasks in detail, but for an in-depth discussion, read my article Realtime and cgroups - a Cautionary Tale[4].
In brief: Real-time tasks require their destination cgroup to have an allocated real-time bandwidth. A global quota divided among all cgroups and their descendants. Improper migration can prevent real-time tasks from running, especially if the target cgroup lacks the necessary RT quota.
Example scenario
Suppose you want to:
- Move all tasks created by the
databaseuser to the DB cgroup - Move all other tasks to the
otherscgroup
Your rules might looks like this:
database * DB
* * others
If this setup includes a mix of normal and real-time tasks, cgrulesengd does not check whether the destination cgroup has real-time bandwidth. It will migrate all tasks based on the rules in /etc/cgrules.conf, placing the burden on the administrator or application to ensure proper RT configuration. This defeats the purpose of automation and reintroduces the same risks the daemon is meant to avoid.
Solution: ignore_rt
To address this, libcgroup 3.2 introduced the ignore_rt option. It teaches cgrulesengd to skip migrating real-time tasks, preserving their ability to meet strict scheduling requirements. For example, if a real-time task is located in a database.realtime cgroup, then it will not be moved to the others cgroup because of the ignore_rt flag.
Updated example with ignore_rt:
database * DB
* * others ignore_rt
This ensures that real-time tasks remain in their original cgroups, where proper bandwidth and scheduling constraints are already configured.
Conclusion
Task classification is a critical, yet often overlooked, aspect of effective cgroup management. While tools like cgrulesengd make automation easier, they must be configured carefully, especially when handling real-time (RT) tasks. Misclassifying or blindly migrating RT processes can lead to failed scheduling, degraded performance, or even system crashes.
The ignore_rt option, introduced in libcgroup 3.2, provides a simple but powerful safeguard. By instructing cgrulesengd to skip real-time tasks, it helps maintain system stability without sacrificing the benefits of automated task placement for non-RT workloads.
If you rely on cgrulesengd for cgroup rule enforcement and your workloads include real-time processes, enabling ignore_rt is not just a good idea but essential. A single flag can mean the difference between efficient automation and unpredictable system behavior.