Книга: Embedded Linux Primer: A Practical, Real-World Approach

17.2.2. Preemption Models

17.2.2. Preemption Models

The first solution to kernel preemption was to place checks at strategic locations within the kernel code where it was known to be safe to preempt the current thread of execution. These locations included entry and exit to system calls, release of certain kernel locks, and return from interrupt processing. At each of these points, code similar to Listing 17-2 was used to perform preemption.

Listing 17-2. Check for Preemption a la Linux 2.4 + Preempt Patch

...
/*
* This code is executed at strategic locations within
* the Linux kernel where it is known to be safe to
* preempt the current thread of execution
*/
if (kernel_is_preemptable() && current->need_resched) preempt_schedule();
...
/*
* This code is in .../kernel/sched.c and is invoked from
* those strategic locations as above
*/
#ifdef CONFIG_PREEMPT
asmlinkage void preempt_schedule(void) {
 while (current->need_resched) {
  ctx_sw_off();
  current->state |= TASK_PREEMPTED;
  schedule();
  current->state &= ~TASK_PREEMPTED;
  ctx_sw_on_no_preempt();
 }
}
#endif
...

The first snippet of code in Listing 17-2 (simplified from the actual code) is invoked at those strategic locations described earlier, where it is known that the kernel is safe to preempt. The second snippet of code in Listing 17-2 is the actual code from an early Linux 2.4 kernel with the preempt patch applied. This interesting while loop causes a context switch via the call to schedule() until all requests for preemption have been satisfied.

Although this approach led to reduced latencies in the Linux system, it was not ideal. The developers working on low-latency soon realized the need to "flip the logic." With earlier preemption models, we had this:

• The Linux kernel was fundamentally nonpreemptable.

• Preemption checks were sprinkled around the kernel at strategic locations known to be safe for preemption.

• Preemption was enabled only at these known-safe points.

To achieve a further significant reduction in latency, we want this in a preemptable kernel:

• The Linux kernel is fully preemptable everywhere.

• Preemption is disabled only around critical sections.

This is where the kernel developers have been heading since the original preemptable kernel patch series. However, this is no easy task. It involves poring over the entire kernel source code base, analyzing exactly what data must be protected from concurrency, and disabling preemption at only those locations. The method used for this has been to instrument the kernel for latency measurements, find the longest latency code paths, and fix them. The more recent Linux 2.6 kernels can be configured for very low-latency applications because of the effort that has gone into this "lock-breaking" methodology.

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