IntroductionMontaVista has developed a hard real-time fully preemptable Linux kernel, based on Linux kernel 2.4. The preemptable kernel has the potential to dramatically improve application responsiveness of the Linux kernel, while fully preserving the standard Linux programming model.

The current prototype of the preemptable Linux kernel (for IA32/X86 platforms) is available for ftp download, at ftp://ftp.mvista.com.

Preemption model

The preemption model used is to allow the kernel to be preempted at any time when it is not locked. This is very different from the model where preemption is actively requested by the currently executing code. Using this model, when an event occurs that causes a higher priority task to be executable, the system will preempt the current task and run the higher priority task. Of course, there are times when this should not be done. These include:

• While handling interrupts
  
  
• While doing "bottom half" processing. Bottom half processing is work that an interrupt routine needs to do, but which can be done at a more relaxed pace.
  
  
• While holding a spinlock, writelock, or readlock. These locks were put in the kernel to protect it from other processors in Symmetric Multiprocessing (SMP) Systems. While these locks are held, the kernel is not preemptable for reentrancy or data protection reasons (just as in the SMP case).
  
  
• While the kernel is executing the scheduler itself. The scheduler is charged with executing the "best" task and if it is engaged in making that decision, it should not be confused by asking it to give up the processor.

At all other times the MontaVista algorithm allows preemption. Also, whenever the system exits from one of the above states, a test is made to see if preemption is called for. If so, the current task is preempted. (Please see the final paragraph below for important comments on this attribute.)

MontaVista's effort to bring preemption to Linux focused on two changes.

First the basic interrupt entry and completion code "entry.S" and its helpers was modified to ensure that it never returns to a user with a pending soft interrupt, context switch, or signal delivery. It was also modified to never return to system code with a pending soft interrupt, or allowed context switch pending. Here "allowed" means that the preemption lock count is zero. The preemption lock count is incremented whenever a spinlock, writelock, readlock, interrupt or trap is taken and decremented when ever these conditions clear.

The second change centered on modifying the header files for spinlock, writelock, readlock, and some SMP code to produce code that modified the preemption count.

The scheduler was changed only slightly to directly prevent its own preemption and to honor an additional "state" TASK_PREEMPTING flag. This flag is set whenever preemption is taken and tells the scheduler that the task is to be treated as running, even though its actual state may be other than running. This allows preemption to occur during wake_up set up times when the kernel sets the current tasks state to something other than running and then does other set up work on the way to calling the scheduler. By using the TASK_PREEMPTING flag on the state (the underlying state is preserved) the scheduler can distinguish between the preemption call and the completion of the wake_up set up. Without this flag, the task could be put to sleep prior to completion of the wake_up setup, and thus would never wake_up.

Continuing development

MontaVista is continuing to work on preemption and is currently looking at the following:

• Change the spinlock, writelock, and readlock macros that also disable interrupts to not bother with the preemption counter. The interrupt system being off prevents preemption, so the flag is not needed.
  
  
• Set up the spinlock, writelock, and readlock macros so that both SMP and preemption can be turned on at the same time. There is no reason an SMP system cannot be preemptable.
  
  
• Measure the impact on throughput of a variety of workloads. Note that under combined I/O and CPU intensive workloads, throughput may actually improve.
  
  
• Do timing analysis on the system locks.
  
  
• Using the above analysis for short locks, consider using the interrupt system as a lock, instead of the preemption counter. This will reduce system overhead.
  
  
• Again, using the above analysis, consider ways to reduce the longer times. This would most likely entail using mutex locks to protect the regions, thus blocking only code that needed the same function, rather than the whole processor. These mutex locks would be based on priority-inheriting binary semaphores. (Priority-inheriting semaphores are designed such that a task holding such a semaphore runs at, or above, the priority of the highest priority task waiting for the semaphore. This prevents a low priority task from being blocked by priority while it is holding a semaphore that a higher priority task wants.)

Over the long term, MontaVista is investigating whether preemption locks can be eliminated (or at least greatly reduced in number) by protecting all the short-duration critical regions with spinlocks that also disable interrupts on the local CPU, and the long-duration critical regions with mutex locks. ("Long duration" means much greater than the time taken by two context switches.) This will reduce the overhead of the preemptable kernel, since there will no longer be any need to test for preemption ("polling for preemption") at the end of a preemption-locked region (which could happen tens of thousands of times per second on an average system). Instead, preemption would happen automatically as part of the interrupt servicing that causes a higher-priority process to become runnable ("event-driven preemption"). Typically, this only happens a few times to a few tens of times per second with an average system workload, making the "event-driven preemption" model much more efficient than the "polling for preemption" model. This method also has an added efficiency in that the system will take advantage of the cache disruption caused by the interrupt (which is unavoidable) to continue with the preemption.

Related stories:
   MontaVista unveils fully preemptable Linux kernel prototype
   MontaVista unveils transparent real-time scheduler for Linux
   The Linux Real-Time Characterization Project
   MontaVista Partners with FSMLabs for RTLinux

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