RT kernel fundamentals - Qualcomm Dragonwing Documentation

A real-time (RT) system provides deterministic, bounded response times to external events. A system qualifies as real-time if it is free of unbounded latency, its worst-case response time can be calculated with precision, and all tasks meet their scheduling deadlines within defined jitter limits. Qualcomm^® Linux ships an LTS RT kernel (6.18.x) built on top of the upstream PREEMPT_RT patch set. The RT kernel is targeted at kernel-space processes that require deterministic scheduling. User-space applications cannot achieve RT behavior by using the RT kernel alone; they must also be written with RT scheduling policies (SCHED_FIFO / SCHED_RR) and pinned to isolated CPU cores.

How PREEMPT_RT changes the kernel

The standard Linux kernel (CONFIG_PREEMPT or CONFIG_PREEMPT_VOLUNTARY) allows certain critical sections to run non-preemptibly, causing unbounded scheduling latency spikes. PREEMPT_RT eliminates this by:

Change	Effect
Converting spinlocks to sleeping mutexes (RT mutexes)	Critical sections become preemptible, reducing worst-case latency
Making interrupt handlers threaded by default	ISRs run as kernel threads with a configurable priority, removing them from the non-preemptible interrupt context
Serialising RCU (Read-Copy-Update) callbacks	RCU callbacks run in dedicated threads rather than softirq context, reducing jitter
Converting `local_lock_t` to per-CPU sleeping locks	Per-CPU code paths become preemptible

The result is a kernel where nearly every code path can be preempted by a higher-priority RT task, giving deterministic worst-case latencies in the microsecond range on tuned Qualcomm hardware. For the authoritative upstream documentation, see Linux kernel realtime.

Required Kconfig options

The Qualcomm Linux meta-qcom layer enables CONFIG_PREEMPT_RT by default through the rt.config configuration fragment included in the linux-qcom-rt_6.18.bb recipe. The following table describes the key Kconfig options and their role: Table: RT kernel Kconfig options

Option	Default in RT build	Description
`CONFIG_PREEMPT_RT`	`y`	Enables full kernel preemption. Mandatory for RT.
`CONFIG_NO_HZ_COMMON`	`y`	Infrastructure for tickless idle and full-tickless operation. Required by `NO_HZ_FULL`.
`CONFIG_NO_HZ_FULL`	`y`	Suppresses scheduling-clock interrupts on CPUs with a single runnable task. Reduces jitter on isolated RT cores.
`CONFIG_CPUSETS`	`y`	Allows grouping CPUs into sets for affinity control. Required for isolating RT tasks to dedicated cores.
`CONFIG_CPU_ISOLATION`	`y`	Enables `isolcpus=` kernel parameter support to remove cores from the general scheduler.
`CONFIG_HZ_1000`	`y`	Sets the kernel timer frequency to 1000 Hz (1 ms tick period). Higher `HZ` values increase scheduling granularity at the cost of more timer overhead.

Verify that these options are active on a running RT system:

zcat /proc/config.gz | grep -E "CONFIG_PREEMPT|CONFIG_NO_HZ|CONFIG_CPUSETS"

Expected output:

# CONFIG_PREEMPT_NONE is not set
# CONFIG_PREEMPT_VOLUNTARY is not set
# CONFIG_PREEMPT is not set
CONFIG_PREEMPT_RT=y
CONFIG_NO_HZ_COMMON=y
CONFIG_NO_HZ_FULL=y
CONFIG_CPUSETS=y

RT vs standard kernel trade-offs

Choosing the RT kernel involves accepting specific trade-offs:

Dimension	Standard kernel	RT kernel
Worst-case scheduling latency	Unbounded (μs to ms)	Bounded (low μs on tuned hardware)
Throughput	Higher, fewer context switches	Slightly lower, more preemption points
Power consumption	Lower in idle	Higher, `NO_HZ_FULL` keeps RT cores active
Debugging complexity	Standard tools	RT-specific tools (`cyclictest`, `ftrace` with `hwlat_detector`)
Use case	General-purpose, multimedia	Industrial control, robotics, time-sensitive networking

The RT kernel is supported on all Qualcomm Linux hardware platforms (QCS6490, IQ-9075, IQ-8275, IQ-615). Latency numbers depend on the specific SoC, CPU topology, and tuning applied. Always measure on target hardware and do not rely solely on simulation results.

Verify RT is active

After booting the RT kernel, confirm full RT preemption is enabled:

# Kernel version string includes -rt suffix
uname -r
# Example: 6.18.0-rt1+

# Confirm PREEMPT_RT in the version banner
cat /proc/version

# Verify the Kconfig flag
zcat /proc/config.gz | grep CONFIG_PREEMPT_RT
# Expected: CONFIG_PREEMPT_RT=y

​How PREEMPT_RT changes the kernel

​Required Kconfig options

​RT vs standard kernel trade-offs

​Verify RT is active

How PREEMPT_RT changes the kernel

Required Kconfig options

RT vs standard kernel trade-offs

Verify RT is active