{"id":18125,"date":"2026-10-05T03:55:57","date_gmt":"2026-10-05T09:55:57","guid":{"rendered":"https:\/\/www.supportpro.com\/blog\/?p=18125"},"modified":"2026-10-05T03:57:27","modified_gmt":"2026-10-05T09:57:27","slug":"rcu-stalls-in-linux-causes-diagnostics-and-recovery","status":"publish","type":"post","link":"https:\/\/www.supportpro.com\/blog\/rcu-stalls-in-linux-causes-diagnostics-and-recovery\/","title":{"rendered":"RCU Stalls in Linux: Causes, Diagnostics and Recovery"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">Linux systems are designed to handle enormous numbers of concurrent operations, and one of the mechanisms that makes this possible is Read-Copy-Update (RCU). RCU allows readers to access shared data efficiently while updates happen safely in parallel. It is widely used inside the Linux kernel and is particularly important on systems with many CPU cores.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">However, when the kernel reports an RCU stall, it means that an RCU grace period is taking much longer than expected. Typical messages include rcu_sched detected stalls, rcu_preempt detected stalls, or warnings that a CPU has been waiting too long.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">An RCU stall does not necessarily mean that RCU itself is broken. More often, it is a symptom of a CPU, interrupt, scheduling, locking, or kernel workload problem. Understanding how to diagnose these stalls is therefore essential for systems engineers and kernel administrators.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">What Is an RCU Stall?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">RCU allows multiple readers to operate without traditional locking overhead. When the kernel needs to modify an RCU-protected object, it waits for a grace period during which all relevant readers have passed through safe states.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">An RCU stall occurs when the kernel cannot complete this grace period within the expected time.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The kernel watchdog may produce messages similar to:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">INFO: rcu_sched detected stalls on CPUs\/tasks<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">or:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">rcu: INFO: rcu_preempt detected stalls on CPUs\/tasks<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The message may identify a CPU, task, or both.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The important point is that an RCU stall is usually an indication of something preventing forward progress, rather than a standalone root cause.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Common Causes of RCU Stalls<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">1. CPU Starvation<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">A CPU that spends too long executing kernel code without reaching an RCU quiescent state can prevent a grace period from completing.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This can happen with:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>CPU-intensive kernel workloads<\/li>\n\n\n\n<li>Long-running kernel loops<\/li>\n\n\n\n<li>Real-time workloads<\/li>\n\n\n\n<li>CPU isolation or affinity problems<\/li>\n\n\n\n<li>Excessive interrupt processing<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">On heavily loaded systems, check whether one CPU is behaving differently from the others.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">2. Interrupts Disabled for Too Long<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Kernel code sometimes disables interrupts to protect critical sections. If interrupts remain disabled for an unexpectedly long period, important kernel activity can be delayed.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Long interrupt-disabled sections are particularly problematic because they can interfere with scheduling, timers, and other mechanisms needed for RCU progress.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Kernel debugging tools such as ftrace, lockdep, and latency tracers can help identify excessive interrupt-disabled time.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">3. Soft Lockups and Hard Lockups<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">RCU stalls frequently appear alongside other kernel warnings.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A soft lockup occurs when a CPU remains busy in kernel mode for too long without allowing the scheduler to make expected progress.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A hard lockup is more severe and generally indicates that a CPU has stopped responding to expected interrupt activity.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">If an RCU stall occurs alongside soft-lockup or hard-lockup messages, investigate those symptoms as part of the same incident.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">4. Long-Running Kernel Code<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Drivers, file systems, networking code, storage subsystems, and other kernel components can sometimes execute operations that take longer than expected.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A problematic driver can therefore produce an RCU stall even though RCU itself is functioning normally.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Pay particular attention to changes made shortly before the problem began, including kernel upgrades, driver updates, firmware changes, or new hardware.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">5. Virtualization Problems<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Virtual machines can experience RCU stalls when the guest vCPUs are not receiving sufficient execution time from the hypervisor.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Possible contributors include:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Host CPU contention<\/li>\n\n\n\n<li>Excessive vCPU over commitment<\/li>\n\n\n\n<li>CPU scheduling delays<\/li>\n\n\n\n<li>Host-level interrupts<\/li>\n\n\n\n<li>Incorrect virtual CPU configuration<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">When an RCU stall occurs inside a virtual machine, investigate both the guest and the underlying host.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Diagnosing an RCU Stall<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Start by collecting the complete kernel log around the event.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">On systems using systemd, commands such as:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">journalctl -k<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">can provide kernel messages.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Traditional systems may use:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">dmesg -T<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Search for RCU messages along with related warnings:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">dmesg -T | grep -i rcu<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Do not examine RCU messages in isolation. Look immediately before and after them for indications of lockups, blocked tasks, I\/O errors, memory pressure, or driver failures.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Examine CPU Activity<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Check CPU utilization and per-CPU behavior using tools such as:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">top<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">or:<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">mpstat -P ALL 1<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A system-wide CPU average can hide a problem affecting only one processor.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Look for CPUs that are consistently saturated or behaving differently from their peers.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Also examine the process responsible for unusually high CPU consumption.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Capture Kernel Stack Traces<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">An RCU stall warning often includes stack information. This can be extremely valuable.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The stack trace can indicate whether the affected CPU is spending time in:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Networking code<\/li>\n\n\n\n<li>Storage drivers<\/li>\n\n\n\n<li>File systems<\/li>\n\n\n\n<li>Interrupt handling<\/li>\n\n\n\n<li>Synchronization primitives<\/li>\n\n\n\n<li>Virtualization-related code<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">If the problem is reproducible, kernel tracing tools can provide additional information about where time is being spent.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Check for Recent Changes<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A reliable troubleshooting process includes a timeline.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Ask:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Was the kernel recently upgraded?<\/li>\n\n\n\n<li>Did a driver change?<\/li>\n\n\n\n<li>Was new hardware introduced?<\/li>\n\n\n\n<li>Did workload characteristics change?<\/li>\n\n\n\n<li>Were CPU affinity or isolation settings modified?<\/li>\n\n\n\n<li>Did virtualization settings change?<\/li>\n\n\n\n<li>Did the problem begin after a configuration deployment?<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">If the timing strongly correlates with a change, compare the affected kernel, driver, or configuration with the previous known-good state.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Recovery Options<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">The appropriate recovery depends on the severity and frequency of the stall.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For an isolated warning where the system remains healthy, monitor the machine and collect additional diagnostics.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">If stalls repeatedly cause severe performance degradation, investigate the responsible kernel subsystem before making changes.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Potential remediation may include:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Updating to a kernel containing relevant fixes<\/li>\n\n\n\n<li>Updating problematic drivers or firmware<\/li>\n\n\n\n<li>Correcting CPU affinity or isolation settings<\/li>\n\n\n\n<li>Reducing excessive CPU or interrupt load<\/li>\n\n\n\n<li>Adjusting virtualization configuration<\/li>\n\n\n\n<li>Removing a recently introduced problematic component<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">Avoid changing multiple variables simultaneously. Controlled changes make it easier to determine which action actually resolved the problem.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">When a Reboot Is Necessary<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">If the system becomes unresponsive, a reboot may ultimately be required. Before doing so, collect as much evidence as possible.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Capture:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Kernel logs<\/li>\n\n\n\n<li>RCU warnings<\/li>\n\n\n\n<li>CPU utilization<\/li>\n\n\n\n<li>Process information<\/li>\n\n\n\n<li>Stack traces<\/li>\n\n\n\n<li>Memory statistics<\/li>\n\n\n\n<li>I\/O statistics<\/li>\n\n\n\n<li>Recent configuration changes<\/li>\n\n\n\n<li>Monitoring data<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">If the system is still partially responsive, gathering this information before rebooting can make the difference between a useful post-incident investigation and a loss of evidence.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Conclusion<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">RCU stalls are important indicators of delayed <a href=\"https:\/\/www.supportpro.com\/blog\/ghostlock-cve-2026-43499-how-to-protect-your-linux-servers-from-the-latest-kernel-vulnerability\/\" target=\"_blank\" rel=\"noopener\">kernel<\/a> progress and should be treated as symptoms of an underlying system condition rather than an immediate indication that RCU itself has failed. CPU starvation, prolonged interrupt-disabled periods, soft or hard lockups, long-running kernel operations, problematic drivers, and virtualization-related scheduling delays can all contribute to an RCU stall.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Effective troubleshooting begins with collecting complete kernel logs and examining the system around the time of the warning. CPU utilization, per-CPU activity, stack traces, I\/O behavior, memory conditions, driver activity, and recent kernel or configuration changes can help identify the source. When the issue is associated with a recent update or hardware change, comparing the current environment with a known-good configuration can also help narrow down the cause.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Recovery should focus on addressing the underlying condition rather than simply suppressing the warning. Kernel, driver, firmware, CPU configuration, workload, or virtualization adjustments may be required depending on the findings. If the system becomes unresponsive and a reboot is necessary, collecting available diagnostic information beforehand can preserve valuable evidence for further investigation. With systematic monitoring and controlled troubleshooting, RCU stalls can be diagnosed more effectively, and recurring kernel performance issues can be reduced.<\/p>\n\n\n\n<div class=\"wp-block-media-text alignwide has-media-on-the-right is-stacked-on-mobile is-vertically-aligned-center has-white-background-color has-background\"><div class=\"wp-block-media-text__content\">\n<p class=\"has-large-font-size wp-block-paragraph\">Facing issues? <\/p>\n\n\n\n<p class=\"has-large-font-size wp-block-paragraph\">Our technical support<br>engineers can solve it. <\/p>\n\n\n\n<!--HubSpot Call-to-Action Code --><span class=\"hs-cta-wrapper\" id=\"hs-cta-wrapper-3350a795-db50-482f-9911-301930d1b1be\"><span class=\"hs-cta-node hs-cta-3350a795-db50-482f-9911-301930d1b1be\" id=\"hs-cta-3350a795-db50-482f-9911-301930d1b1be\"><!--[if lte IE 8]><div id=\"hs-cta-ie-element\"><\/div><![endif]--><a href=\"https:\/\/cta-redirect.hubspot.com\/cta\/redirect\/2725694\/3350a795-db50-482f-9911-301930d1b1be\" ><img decoding=\"async\" class=\"hs-cta-img\" id=\"hs-cta-img-3350a795-db50-482f-9911-301930d1b1be\" style=\"border-width:0px;\" src=\"https:\/\/no-cache.hubspot.com\/cta\/default\/2725694\/3350a795-db50-482f-9911-301930d1b1be.png\"  alt=\"Contact Us today!\"\/><\/a><\/span><script charset=\"utf-8\" src=\"https:\/\/js.hscta.net\/cta\/current.js\"><\/script><script type=\"text\/javascript\"> hbspt.cta.load(2725694, '3350a795-db50-482f-9911-301930d1b1be', {\"useNewLoader\":\"true\",\"region\":\"na1\"}); <\/script><\/span><!-- end HubSpot Call-to-Action Code -->\n<\/div><figure class=\"wp-block-media-text__media\"><img fetchpriority=\"high\" decoding=\"async\" width=\"904\" height=\"931\" src=\"https:\/\/www.supportpro.com\/blog\/wp-content\/uploads\/2022\/09\/Free-server-checkup.png\" alt=\"guy server checkup\" class=\"wp-image-12943 size-full\" srcset=\"https:\/\/www.supportpro.com\/blog\/wp-content\/uploads\/2022\/09\/Free-server-checkup.png 904w, https:\/\/www.supportpro.com\/blog\/wp-content\/uploads\/2022\/09\/Free-server-checkup-291x300.png 291w, https:\/\/www.supportpro.com\/blog\/wp-content\/uploads\/2022\/09\/Free-server-checkup-768x791.png 768w, https:\/\/www.supportpro.com\/blog\/wp-content\/uploads\/2022\/09\/Free-server-checkup-585x602.png 585w\" sizes=\"(max-width: 904px) 100vw, 904px\" \/><\/figure><\/div>\n","protected":false},"excerpt":{"rendered":"<p>Linux systems are designed to handle enormous numbers of concurrent operations, and one of the mechanisms that makes this possible is Read-Copy-Update (RCU). RCU allows readers to access shared data&hellip;<\/p>\n","protected":false},"author":39,"featured_media":18126,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_lmt_disableupdate":"","_lmt_disable":"","footnotes":""},"categories":[91],"tags":[],"class_list":["post-18125","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-linux"],"aioseo_notices":[],"aioseo_head":"\n\t\t<!-- All in One SEO Pro 5.0.1.1 - aioseo.com -->\n\t<meta name=\"description\" content=\"Learn how to diagnose and recover from Linux RCU stalls, including CPU starvation, lockups, driver issues, virtualization problems, and kernel warnings.\" \/>\n\t<meta name=\"robots\" content=\"max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n\t<meta name=\"author\" content=\"Anjali Sindhu\"\/>\n\t<link rel=\"canonical\" href=\"https:\/\/www.supportpro.com\/blog\/rcu-stalls-in-linux-causes-diagnostics-and-recovery\/\" 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