Rcu_sched Self-Detected Stall On CPU

Have you ever encountered the “rcu_sched self-detected stall on CPU” error and felt completely lost? Don’t worry, you’re not alone.

The “rcu_sched self-detected stall on CPU” error indicates a delay in the CPU’s RCU mechanism, which can cause potential system slowdowns and SSH inaccessibility. It often results from high system load or hardware issues. Regular maintenance helps prevent these stalls.

In this article, we’ll break down this complex topic into bite-sized pieces, making it easier to understand and address.

Table of Contents

Understanding rcu_sched

Before diving into the issue, let’s clarify what rcu_sched is. The term rcu_sched stands for Read-Copy-Update scheduler.

It’s a synchronization mechanism used in the Linux kernel to handle concurrent read and write operations efficiently.

rcu_sched plays a critical role in maintaining system performance by ensuring that updates do not interfere with ongoing reads.

What is a CPU Stall?

A CPU stall occurs when a processor core becomes idle or stops executing instructions due to waiting on resources or instructions. This can significantly hamper system performance, leading to slowdowns and unresponsiveness.

Using RCU’s CPU Stall Detector

The RCU’s CPU Stall Detector detects when a CPU is too slow to process, causing delays. It shows warning messages to alert users of potential problems, helping to identify and fix system performance issues.

Self-Detected Stalls

Self-detected stalls are a type of CPU stall where the system itself identifies the problem. In the context of rcu_sched, the scheduler has recognized a delay in processing that could affect system stability or performance.

What is RCU?

RCU (Read-Copy-Update) is a synchronization mechanism in the Linux kernel used to manage access to shared data.

It allows multiple readers to access the data concurrently while a writer can make updates without blocking the readers.

The RCU scheduler ensures that updates to data structures are deferred until it is safe to make these changes, thereby improving performance.

Causes of RCU_SCHED Self-Detected Stall

Several factors can contribute to an RCU_SCHED self-detected stall:

High CPU Load: When a CPU is under heavy load, it may not process RCU callbacks promptly, leading to a backlog.
Long-Running Tasks: Tasks that take a long time to complete can block RCU callbacks, causing stalls.
Memory Contention: When multiple processes compete for memory, the CPU may delay handling RCU callbacks.
Kernel Bugs: Certain kernel versions and configurations may have bugs that lead to RCU stalls.
Hardware Issues: Inconsistent behaviour in hardware, such as CPUs or memory, can also cause stalls.

Symptoms of RCU_SCHED Self-Detected Stall

The symptoms of an RCU_SCHED self-detected stall include:

System Slowness: The system may become unresponsive or sluggish.
Dropped Tasks: Tasks may be delayed or dropped, leading to data loss or corruption.
Kernel Panics: The system may experience kernel panics and crashes in severe cases.

Diagnosing RCU_SCHED Stalls

To diagnose RCU_SCHED stalls, you can use several tools and methods:

System Logs: Check the system logs (/var/log/syslog or /var/log/messages) for messages related to RCU stalls.
Perf Tool: The perf tool can monitor the RCU scheduler and identify issues.
rcutorture Tool: This tool stress-tests the RCU subsystem to identify potential problems.

Mitigating and Resolving RCU_SCHED Stalls

There are various approaches to mitigate and resolve RCU_SCHED stalls:

Reduce CPU Load: Reduce the number of tasks or optimize running tasks to decrease CPU load.
Update Kernel: Ensure you are using the latest kernel version, as newer versions often contain fixes for known issues.
Kernel Parameters: Adjust kernel parameters to increase the threshold for detecting stalls. For example, increasing the kernel.watchdog_thresh parameter can help:
sudo sysctl -w kernel.watchdog_thresh=60
Memory Management: Reduce memory contention by optimizing memory usage and freeing up memory.
Contact Experts: If the issue persists, seek assistance from system administrators or kernel developers.

Software Solutions

Software solutions can also help mitigate this issue. Applying kernel patches, using monitoring tools like Nagios or Zabbix, and leveraging system profiling tools can help identify and resolve stalls.

Hardware Considerations

Sometimes, the issue may lie in the hardware. Upgrading your CPU, adding more RAM, or replacing failing components can make a significant difference. Regular hardware diagnostics can preemptively identify potential failures.

Kernel Tuning

Tuning kernel parameters can also help. Adjusting settings like rcu_cpu_stall_timeout can give you more control over how the system handles potential stalls. Understanding and configuring kernel parameters appropriately is critical to a stable system.

Case Studies and Examples

1. VirtualBox and RCU_SCHED Stalls

Users running virtualized environments like VirtualBox have reported RCU_SCHED stalls, especially when multiple CPUs are assigned to virtual machines.

Reducing the number of CPUs allocated to the VM can mitigate the issue. Additionally, enabling high-precision event timers (HPET) in the VM settings has been found to reduce the occurrence of these stalls.

2. Ryzen CPUs and RCU_SCHED Stalls

Users with first and second-generation Ryzen CPUs have reported random RCU_SCHED stalls across kernel versions.

Increasing the watchdog threshold and ensuring the system runs the latest stable kernel can help mitigate these issues.

Interpreting RCU’s CPU Stall-Detector “Splats”

When the RCU detects a slow CPU, it generates messages called “splats.” These messages detail which CPUs are causing delays and include stack traces.

By analyzing these splats, system administrators can identify the problematic areas and address the issues to improve system performance and stability.

Multiple Warnings From One Stall

One CPU detecting a stall can cause other CPUs to issue warnings. This redundancy ensures that even if one CPU misses the issue, others will report it, ensuring performance problems are noticed and addressed quickly.

This mechanism helps maintain system reliability and responsiveness by ensuring prompt detection of stalls.

Stall Warnings for Expedited Grace Periods

Stall warnings for expedited grace periods occur when a CPU fails to meet the shorter timeframes set for certain critical operations.

These warnings highlight issues that need immediate attention to prevent system slowdowns or failures. Addressing these warnings promptly is crucial for maintaining optimal performance and avoiding disruptions in system operations.

Rcu_sched detected stalls on VirtualBox

This error occurs in VirtualBox when a CPU is too slow to process tasks, causing delays and potential system hangs. It can be mitigated by adjusting CPU settings or updating software.

Rcu_sched self-detected stall on CPU Virtualbox

In VirtualBox, this error occurs when a CPU fails to process RCU tasks promptly, leading to system slowdowns. Reducing the number of CPUs assigned to the VM can help fix this issue.

Rcu_preempt self-detected stall on CPU

This error occurs when the RCU-preempt scheduler detects a CPU delay. It indicates that real-time tasks are not being processed quickly enough, which can cause system performance problems.

Rcu_sched self detected stall on CPU centos

This error means that a CPU is not processing RCU tasks fast enough on CentOS systems, leading to delays. Updating the kernel or adjusting system parameters can help resolve this issue.

Rcu_sched self-detected stall on CPU VMware

This error in VMware indicates that a CPU is slow in handling RCU tasks, causing performance issues. It can often be fixed by adjusting VM settings or updating the host system.

Rcu_sched high CPU usage

High CPU usage related to RCU_sched indicates that RCU tasks overburden the CPU. Reducing system load or optimizing task scheduling can help alleviate this issue.

Rcu_sched kthread starved for jiffies

This error means the RCU scheduler thread has been delayed for too many jiffies (time units). This significant delay can cause the RCU system to fall behind in its tasks, leading to performance issues or system instability.

Rcu_sched self-detected stall on CPU + watchdog: BUG: soft lockup

This error indicates a CPU has detected its delay in processing RCU tasks. The watchdog timer triggers a soft lockup warning, which means the system might freeze or become unresponsive due to this delay.

What does ‘self-detected stall on CPU’ syslog message denote on Ubuntu 16?

On Ubuntu 16, this syslog message indicates that a CPU has identified that it is too slow to process RCU tasks. This self-detection helps diagnose potential system performance issues and ensure timely corrective actions.

kernel: INFO: rcu_sched self-detected stall on CPU on Allwinner H3, Ubuntu 16.04.6 LTS 4.14.52

This message means that an Allwinner H3 CPU running Ubuntu 16.04 has detected it is stalling on RCU tasks. This detection highlights potential delays affecting system stability and performance, requiring investigation and resolution.

INFO: Rcu_sched detected stalls on CPUs/tasks

This indicates that the RCU scheduler has found one or more CPUs or tasks causing processing delays. Such detections help identify the specific CPUs or tasks responsible, allowing for targeted troubleshooting and system optimization.

Rcu: INFO: Rcu_sched self-detected stall on CPU

This message means that the RCU scheduler has identified a CPU that is taking too long to process its tasks. This self-detection indicates potential system slowdowns or lag and requires attention to maintain performance.

Ubuntu 14.04.3 startup slow (‘dmesg’: “self-detected stall on cpu”, maybe because of ‘alsa-sink’?)

On Ubuntu 14.04.3, a slow startup accompanied by this message could be linked to audio system issues with ‘alsa-sink’. This problem causes the CPU to delay processing RCU tasks, leading to slower system boot times.

What might cause a single “rcu_sched detected stall on CPU” warning in syslog?

Temporary CPU overload, long-running tasks, or brief system hiccups can cause a single warning. It usually indicates a minor delay in processing RCU tasks and is generally not a severe issue unless it repeats frequently.

“rcu_sched detected stalls on CPUs/tasks” – jiffies – ESXi Ubuntu 16 FileServer Guest

This ESXi Ubuntu 16 FileServer message indicates that the virtual CPUs took too long (measured in jiffies) to handle RCU tasks. This delay can cause system performance issues and requires VM settings or resource allocationadjustment.

Errors – Not Booting: RCU_SCHED SELF-DETECTED STALL ON CPU

If the system fails to boot and shows this error, a CPU is significantly stalling on RCU tasks, preventing the system from starting correctly. This could be due to severe system configuration or hardware issues that need urgent attention.

Rcu_sched Self-Detected Stall – Is It A Watchdog?

Yes, it involves the watchdog timer. When the RCU scheduler detects a stall, it triggers the watchdog timer, which helps identify and report the delay to prevent system hang-ups and maintain stability.

What is this Error? rcu_sched self-detected stall on CPU

This error means a CPU has identified a delay in processing RCU tasks, which can lead to system slowdowns or instability. It signals that the CPU is not handling tasks efficiently.

Proxmox 8.1 – kernel 6.5.11-4 – rcu_sched stall CPU

In Proxmox 8.1 with kernel 6.5.11-4, this error indicates that the CPU is taking too long to process RCU tasks. This can cause performance issues and might require kernel updates or configuration adjustments.

Rcu_sched Self-Detected Stall On Cpu During The Backup

This error during a backup means the CPU is delayed in processing RCU tasks, likely due to high resource usage from the backup process. It can cause system slowdowns and affect backup performance.

Do I need to worry about CPU stall warnings?

Yes, these warnings indicate that your CPU is not processing tasks efficiently, which can lead to system performance issues. Monitoring and addressing these warnings can help maintain system stability.

RCU CPU Stall Warnings

These warnings mean that a CPU is taking too long to handle RCU tasks, which can lead to performance problems. Investigating and resolving these warnings is essential to ensure the system runs smoothly.

FAQs

1. What is a RCU stall?

An RCU stall happens when the CPU takes too long to handle RCU tasks, causing delays and potential system performance issues.

2. What is rcu_sched in Linux?

rcu_sched in Linux is a variant of RCU that handles scheduling and ensures tasks are processed efficiently without causing delays or stalls.

3. What is rcu_nocbs?

rcu_nocbs is a kernel parameter that offloads RCU callback processing to separate threads, reducing CPU load and improving system performance.

4. What is RCU cpu?

RCU CPU refers to the CPU’s role in handling Read-Copy-Update (RCU) tasks, which manage access to shared data without blocking other operations.

5. What is the use of RCU in Linux?

RCU is used in Linux to manage access to shared data structures. It allows multiple readers and writers to operate without blocking each other, thus improving performance.

6. How does stall detection work?

Stall detection works by monitoring CPU activity and identifying when a CPU is taking too long to process tasks. This triggers warnings to help diagnose and fix performance issues.

7. What is RCU used for?

RCU is used to efficiently synchronize the Linux kernel, allowing concurrent read and write operations on shared data structures without locking.

8. What is the RCU configuration?

RCU configuration involves setting kernel parameters to optimize RCU performance, such as rcu_nocbs for offloading callbacks and router settings for tuning RCU behaviour.

9. How many RCU do you need?

The number of RCUs needed depends on the system’s workload and configuration. One RCU per CPU or a balanced configuration based on performance requirements is ideal.

10. ERL: rcu_sched self-detected stall on CPU

This error means a CPU has identified a delay in processing RCU tasks, signalling potential performance issues and needing corrective action.

Conclusion

Regular maintenance and monitoring can prevent the “rcu_sched self-detected stall on CPU” error, which causes system slowdowns and SSH inaccessibility. Address high CPU load and optimize tasks to mitigate this issue.