Frequently Asked Questions

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Questions are given by:

Jaswinder Singh

Answers are given by:

Luotao Fu (l.fu AT pengutronix DOT de), Pengutronix e.K., Kernel Development Group
Robert Schwebel (r.schwebel AT pengutronix DOT de), Pengutronix e.K., Kernel Development Group
Remy Bohmer


Revision History
Revision 1 2007-09-29
Draft / Work in progress


FAQ for realtime support in Linux Kernel


Question Index

1. Realtime Support questions

2. Architecture questions

3. Mailing list questions

4. Realtime Patches questions

5. Configuring/compiling questions

6. Realtime samples/Performance questions

7. Realtime Applications questions

8. Maintainers questions

Section 1 - Realtime Support questions

Q1: What is realtime support


Q2: How realtime works

A2: The RT-Preempt patch converts Linux into a fully preemptible kernel. The magic is done with:

  • Making in-kernel locking-primitives (using spinlocks) preemptible though reimplementation with rtmutexes:
  • Critical sections protected by i.e. spinlock_t and rwlock_t are now preemptible. The creation of non-preemptible sections (in kernel) is still possible with raw_spinlock_t (same APIs like spinlock_t)
  • Implementing priority inheritance for in-kernel spinlocks and semaphores. For more information on priority inversion and priority inheritance please consult Introduction to Priority Inversion
  • Converting interrupt handlers into preemptible kernel threads: The RT-Preempt patch treats soft interrupt handlers in kernel thread context, which is represented by a task_struct like a common userspace process. However it is also possible to register an IRQ in kernel context.
  • Converting the old Linux timer API into separate infrastructures for high resolution kernel timers plus one for timeouts, leading to userspace POSIX timers with high resolution.

Q3: What is the procedure to add realtime support

A3: Download latest realtime preemption patch from then get related vanilla kernel from then patch, configure and compile

Section 2 - Architecture questions

Q1: Which Architecture are supporting realtime and who is maintainer


Processor Supported Contact(s)

Q2: What is the procedure to support realtime in new architecture


Section 3 - Mailing list questions

Q1: realtime mailing list and archives


Q2: If I have some doubts or problems regarding realtime, do I need to send mail in realtime mailing list or linux kernel mailing list or both


Section 4 - Realtime Patches questions

Q1: Why I need to download realtime patches, If realtime support is in Linux kernel


Q2: Download latest realtime patches


Q3: Where can I post my realtime patches

A3: Please send patches for the CONFIG_PREEMPT_RT Patch Set to LKML and put Ingo Molnar and Thomas Gleixner on CC.

Please do not send clocksource and clockevents related patches against the -rt patch. Make sure they apply against the latest -hrt-dyntick patch. -hrt-dyntick might be a bit ahead of -rt at times, but the -rt patch pulls -hrt-dyntick on a regular base.

Section 5 - Configuring/compiling questions

Q1: How to enable/disable realtime support


Q2: Change realtime support run time or statically


Q3: Required parameters for configuring realtime kernel


Q4: Optional parameters for configuring realtime kernel, but they effect realtime performance


Section 6 - Realtime samples/Performance questions

Q1: What effects by realtime support and how much and samples to test it and who maintains these samples a. scheduling, task switching time b. Interrupt latency c. Interprocess communication d. Filesystem e. Device drivers f. Network g. X-Windows


Section 7 - Realtime Applications questions

Q1: List of realtime APIs


Q2: How to write realtime applications


Q3: Which programming languages are suitable for writing realtime applications


Q4: Keep following things in mind while writing realtime applications

A4: Taking care of the following during the initial startup phase:

  • Call directly from the main() entry the mlockall() call.
  • Create all threads at startup time of the application, and touch each page of the entire stack of each thread. Never start threads dynamically during RT show time, this will ruin RT behavior.
  • Never use system calls that are known to generate pagefaults, such as fopen(). (Opening of files does the mmap() system call, which generates a page-fault).
  • Do not use 'compile time static arrays' without initializing them directly after startup, before RT show time.

Q5: Do I need to recompile my applications to get realtime performance


Section 8 - Maintainers questions

Q1: Maintainers of realtime kernel patch


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