The challenges of the stability of the Linux scheduler in the face of performance regressions since 2024
Critical fixes have recently been deployed for the Linux system scheduler, aimed at remedying performance regressions which have increased since the release of the Linux 6.11 kernel in September 2024. These concerns arise in a context where the use of major distributions such as Ubuntu, Fedora, or Debian has been impacted by a notable drop in system responsiveness, particularly in the management of intensive tasks or multi-core configurations. Developers and systems engineers have seen degradation of up to 5-10% in some critical benchmarks, making it difficult to operationally manage many servers in production. The complexity of the scheduler code, coupled with sometimes poorly calibrated optimizations, was at the heart of these problems. The recent update, deployed in the form of RFC (Request for Comments) patches, marks a step in the attempt to restore optimal behavior. The latter is based on a series of five fixes developed by Linux engineer Peter Zijlstra, in collaboration with other contributors to the open source ecosystem.
The major issues concern stability and performance, especially for distributions like OpenSUSE and Mandriva, which rely on the Linux kernel for demanding systems. The problem results from a complex evolution of the scheduler, integrated in version 6.15-rc4, which introduced changes to improve workload management, but which sometimes generated unexpected side effects. This context has pushed the community to prioritize the correction of these regressions, because performance is a key factor for the competitiveness of distributions, particularly in the context of enterprise deployments or on cloud servers. The need to maintain compatibility with various architectures, including ARM or x86 configurations of different generations (for example, AMD Ryzen or Intel Ice Lake), further complicates the rapid resolution of these issues. The technical communication around this update shows a strong desire for transparency, while emphasizing that some fixes are still being evaluated to ensure their long-term stability. To view the complete list of changes, the link to the RFC repository here remains an essential resource for professionals who want to keep up with these developments. Technical Analysis of Linux Scheduler Performance Fixes Technical Analysis of Linux Scheduler Performance Fixes
Fixes for the Linux scheduler primarily target fundamental aspects of task scheduling and management, including the behavior of CPU cores when allocating resources. The series of patches deployed, currently in the testing phase under the RFC (Request for Comments) label, mainly modifies two key areas: task preemption and cache management of running tasks. These changes are intended to mitigate a drop in performance observed during specific benchmarks, such as *schbench*, developed by Chris Mason as part of regression monitoring on Linux 6.11. In this context, it became critical to ensure better consistency in the management of cores under heavy load. The update notably includes adjustments to the way the kernel reports the status of tasks in transaction, as shown in the patch intended to improve the consideration of the queue of pending tasks, particularly on architectures like Intel Skylake or Sapphire Rapids.
Key Change
Expected Impact
| Improved task management | Reworked `ttwu_stat()` management code | Reduced wakeup times and improved responsiveness |
|---|---|---|
| Optimized scheduling policy | Reorganized queues and prioritization | More consistent system under high load |
| Stability | Fixed core synchronization bugs | Reduced deadlocks and crashes |
| Current Linux scheduler performance challenges on different architectures | The main concern identified in these regressions is the diversity of hardware architectures on which Linux is deployed. Platforms such as those equipped with AMD Ryzen or Intel Xeon processors are particularly affected, but the effects vary depending on the configuration. For example, the benchmark conducted on a server equipped with an Intel Skylake shows performance remaining at approximately 93% of that before version 6.11, a noticeable loss in high-frequency scheduling contexts. On the other hand, for a machine using a Sapphire Rapids processor, the drop is 4 to 5%. The disparity in these results underscores the need to tailor patches to each architecture, taking into account specific cache management or hyperthreaded cores. | Distributions such as Linux Mint and Arch Linux have been at the forefront of testing these patches. Compatibility with older or underperforming systems, particularly those using ARM platforms or more exotic architectures like Mandriva or Solus, represents an additional step in the process. The difficulty is compounded by the fact that modifications must remain minimal to avoid introducing new bottlenecks or unforeseen bugs. The developers in charge of optimization strive to evolve the scheduler while maintaining backward compatibility. A summary table shows the impact on performance measured by different architectures: |
Architecture
Performance Before Correction
Performance After Correction
Difference
| Intel Skylake | 93% | 97% | +4% |
|---|---|---|---|
| Intel Sapphire Rapids | 95% | 99% | +4% |
| AMD Ryzen | 90% | 93% | +3% |
| ARM Cortex | 85% | 87% | +2% |
| Outlook and Implications for Linux System Stability in 2025 | Outlook and Implications for Linux System Stability in 2025 | The patches in testing highlight a critical aspect: the need for the Linux project to strengthen the scheduler’s resilience in the face of hardware diversity and the growing demands of modern environments. Kernel stability, already severely tested by the planned obsolescence of certain components such as Mandriva chips or older processors with limited support, must continue to evolve. The question of the gradual abandonment of certain kernels, particularly those supporting 486 or 586 architectures that no longer support new features, is becoming crucial. The official documentation now indicates that many systems that have exceeded their technical lifecycle will be required to migrate to more recent versions in order to fully benefit from the improvements relayed by these patches. Furthermore, the community is vigilant about the impact of these changes on compatibility with older software or software specific to certain distributions, such as Linux Mint or OpenSUSE. | Looking ahead to 2025, the overall trend is toward a consolidation of the Linux kernel’s performance management capabilities while ensuring increased stability. Linux Inc.’s simple yet clear communication through its various kernel releases, including the early release of Linux 6.15-rc6, demonstrates its commitment to transparency and continuous improvement. Relations with the business world, particularly through Red Hat and other Linux solution providers, remain essential: these players play a key role in capturing feedback from real-world deployments. The gradual withdrawal of support for underperforming or vulnerable architectures, such as the Cortex 486, is a step in the process of refocusing on modern platforms capable of fully exploiting open source advances. For more details, see our dedicated article on the end of support here. |
