Real-Time Fedora Kernel Guide 2026

With the mainline merge of PREEMPT_RT in Linux 6.12 (late 2024), building a real-time Fedora kernel no longer requires applying out-of-tree patches. The RT preemption model is a config option. You have three choices:

# Pre-built RT kernel from COPR (easiest) . # Build via Fedora's fedpkg RPM infrastructure . # Build vanilla from source (most control)

Method A — Pre-Built RT Kernel from COPR

The @kernel-vanilla/stable COPR ships kernel-rt packages for x8664 and aarch64. sudo dnf -y copr enable @kernel-vanilla/stable
sudo dnf install kernel-rt kernel-rt-core kernel-rt-modules kernel-rt-modules-extraVanilla COPR kernels can't be signed for Secure Boot. Check and disable if needed: mokutil --sb-state
sudo mokutil --disable-validation    # then reboot through MOK promptsSet RT kernel as default and reboot: sudo grubby --set-default /boot/vmlinuz-*.rt*
sudo rebootVerify: uname -a                              # look for PREEMPT_RT
cat /sys/kernel/realtime              # should return 1
cat /sys/kernel/debug/sched/preempt   # should show "full"==Method B — Fedora RPM Build with RT== Use this if you need Fedora's own kernel patches with RT enabled, or want custom config. sudo dnf install fedpkg fedora-packager rpmdevtools ncurses-devel pesign grubby
sudo dnf builddep kernel.spec

fedpkg clone -a kernel
cd kernel
git fetch
git switch f44    # or your release branchCreate kernel-local in the kernel source directory to override Fedora's defaults: # kernel-local
CONFIG_PREEMPT_RT=y
CONFIG_HIGH_RES_TIMERS=y
CONFIG_NO_HZ_FULL=y
CONFIG_HZ_1000=y
CONFIG_HZ=1000
CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE=yAlternatively, run make menuconfig and set: General Setup → Preemption Model → Fully Preemptible Kernel (Real-Time) General Setup → Timers subsystem → High Resolution Timer Support General Setup → Timers subsystem → Full dynticks system (tickless) Processor type and features → Timer frequency → 1000 HZ Power management → CPU Frequency scaling → Default governor → performance

Set a unique build ID in kernel.spec:

Build and install:

Method C — Vanilla Source Build (Most Control)

For the absolute latest RT fixes or exotic configurations.

Configure with your running kernel as base:

Critical config options beyond PREEMPTRT: CONFIG_NO_HZ_FULL=y — adaptive tickless on isolated cores CONFIG_HZ=1000 — 1ms timer granularity CPU_FREQ_DEFAULT_GOV_PERFORMANCE=y — prevent downclocking CONFIG_DEBUG_PREEMPT=n — disable debugging overhead CONFIG_SLUB_DEBUG=n — lower latency CONFIG_LOCKDEP=n — huge latency penalty if left on CONFIG_TRANSPARENT_HUGEPAGE=n — avoid non-deterministic TLB flushes CONFIG_NUMA_BALANCING=n — prevent NUMA migration jitter Disable signing keys to avoid build errors: scripts/config --disable SYSTEM_TRUSTED_KEYS
scripts/config --disable SYSTEM_REVOCATION_KEYS
scripts/config --set-str CONFIG_SYSTEM_TRUSTED_KEYS ""
scripts/config --set-str CONFIG_SYSTEM_REVOCATION_KEYS ""Build: make -j$(nproc) bindeb-pkg
sudo make modules_install
sudo make install
sudo grub2-mkconfig -o /boot/grub2/grub.cfg
sudo reboot==Why PREEMPTRT Matters for Music==

Measured on a Ryzen 7 7800X3D with a Focusrite Scarlett 4i4 (USB), running Ardour with 30 plugins at quantum 128/48kHz (linuxdj.com):

RT eliminates the unpredictable latency tail. Threaded IRQs mean your audio thread can preempt interrupt handlers. Sleeping spinlocks become RT-mutexes with priority inheritance. Critical sections that previously disabled preemption are now preemptible — no more millisecond stalls from filesystem journal commits (kernel.org).

Tips & Tricks Nobody Tells You

1 — USB Autosuspend Is the #1 XRun Culprit

USB audio interfaces get put to sleep by the kernel. This causes intermittent crackles:

Persistent udev rule at /etc/udev/rules.d/90-usb-audio-power.rules:

2 — Raise Your Audio Interface's IRQ Thread Priority

PREEMPTRT turns IRQ handlers into threads at SCHED_FIFO 50 by default. PipeWire runs at priority 88. Boost your interface: cat /proc/interrupts | grep -i audio        # find IRQ number (e.g. 34)
chrt -f -p 85 $(pgrep -f "irq/34-")===3 — Disable Transparent HugePages & NUMA Balancing=== These cause non-deterministic latency spikes. Add to kernel cmdline in /etc/default/grub: GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX transparent_hugepage=never numa_balancing=disable"===4 — Isolate CPUs for the Tightest Latency=== Remove cores from the general scheduler: GRUB_CMDLINE_LINUX="$GRUB_CMDLINE_LINUX isolcpus=2,3 nohz_full=2,3 rcu_nocbs=2,3 irqaffinity=0-1"Pin PipeWire to those cores via config, and disable SMT: echo off | sudo tee /sys/devices/system/cpu/smt/control===5 — Use rtla timerlat, Not Just cyclictest=== cyclictest shows you how much latency. rtla timerlat (built into the kernel since 6.x) tells you why: sudo rtla timerlat top -a 150 -Pf:98This captures the blocking thread's stack trace when latency exceeds 150 μs — invaluable for identifying SMIs, drivers, or kernel paths (kernel.org). ===6 — Detect SMIs (System Management Interrupts)=== SMIs are invisible to the kernel and produce 50–300 μs spikes that even PREEMPTRT can't fix:

If detected, disable in BIOS — no kernel tuning helps.

7 — PipeWire RT Priority Without RTKit

RTKit defaults to priority 20, wasting what PREEMPTRT provides. Override with a systemd drop-in at ~/.config/systemd/user/pipewire.service.d/realtime.conf: [Service]
LimitRTPRIO=95
LimitMEMLOCK=infinity
LimitNICE=-20===8 — Disable WiFi & Bluetooth During Sessions=== WiFi power management and Bluetooth radios generate thousands of IRQs per second: iw dev wlan0 set power_save off
rfkill block bluetooth===9 — Don't Chase the Smallest Buffer=== The stability difference between quantum 64 and 128 is enormous, but the latency difference (1.3 ms vs 2.7 ms at 48 kHz) is imperceptible. Start at 256, work down, stop at the first unstable level and step back one (linuxdj.com). ==Quick Verification Reference== Check || Command || Good Result RT kernel || cat /sys/kernel/realtime || 1 Preemption || uname -a || Contains PREEMPT_RT Governor || cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor || performance PipeWire RT || ps -eLo cls,rtprio,comm \|| grep pipewire || FF 88 Memory lock || ulimit -l || unlimited USB autosuspend || cat /sys/bus/usb/devices/*/power/control \|| uniq || on Isolated CPUs || cat /sys/devices/system/cpu/isolated || Your chosen cores WiFi power save || iw dev wlan0 get power_save || off ==Sources== Linux kernel real-time documentation PREEMPT_RT differences from mainline Fedora wiki: custom kernel build Fedora wiki: kernel-vanilla COPR rtla-timerlat documentation linuxdj.com: RT latency measurements linuxdj.com: low-latency audio checklist realtime-linux.org: getting started with PREEMPT_RT proteanos.com: RT latency benchmarks

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