Real-Time Linux Kernel Scheduler
The pull_rt_task() algorithm looks at all the overloaded runqueues in a root domain and checks whether they have a real-time task that can run on the target runqueue (that is, the target CPU is in the task->cpus_allowed_mask) and is of a priority higher than the task the target runqueue is about to schedule. If so, the task is queued on the target runqueue. This search aborts only after scanning all the overloaded runqueues in the root domain. Thus, the pull operation may pull more than one task to the target runqueue. If the algorithm only selects a candidate task to be pulled in the first pass and then performs the actual pull in the second pass, there is a possibility that the selected highest-priority task is no longer a candidate (due to another parallel scheduling operation on another CPU). To avoid this race between finding the highest-priority runqueue and having that still be the highest-priority task on the runqueue when the actual pull is performed, the pull operation continues to pull tasks. In the worst case, this might lead to a number of tasks being pulled to the target runqueue, which would later get pushed off to other CPUs, leading to task bouncing. Task bouncing is known to be a rare occurrence.
Consider the scenario shown in Figure 2. Task T2 is being preempted by task T3 being woken on runqueue 0. Similarly, task T7 is voluntarily yielding CPU 3 to task T6 on runqueue 3. We first consider the scheduling action on CPU 0 followed by CPU 3. Also, assume all the CPUs are in the same root domain. The pri_active bitmap for this system of CPUs will look like Figure 3. The numbers in the brackets indicate the actual priority that is offset by two (as explained earlier).
On CPU 0, the post-schedule algorithm would find the runqueue under real-time overload. It then would initiate a push operation. The first set bit of pri_active yields runqueue of CPU 1 as the lowest-priority runqueue suitable for task T2 to be pushed to (assuming all the tasks being considered are not affined to a subset of CPUs). Once T2 is pushed over, the push algorithm then would try to push T1, because after pushing T2, the runqueue still would be under RT overload. The pri_active after the first operation would be as shown in Figure 4. Because the lowest-priority runqueue has a priority greater than the task to be pushed (T1 of priority 85), the push aborts.
Now, consider scheduling at CPU 3, where the current task of priority 92 is voluntarily giving up the CPU. The next task in the queue is T6. The pre-schedule routine would determine that the priority of the runqueue is being lowered, triggering the pull algorithm. Only runqueues 0 and 1 being under real-time overload would be considered by the pull routine. From runqueue 0, the next highest-priority task T1 is of priority greater than the task to be scheduled—T6, and because T1 < T3 and T6 < T3, T1 is pulled over to runqueue 3. The pull does not abort here, as runqueue 1 is still under overload, and there are chances of a higher-priority task being pulled over. The next highest task, T4 on runqueue 1, also can be pulled over, as its priority is higher than the highest priority on runqueue 3. The pull now aborts, as there are no more overloaded runqueues. The final status of all the runqueues is as shown in Figure 5, which is in accordance with scheduling requirements on real-time systems.
Although strict priority scheduling has been achieved, runqueue 3 is in an overloaded state due to the pull operation. This scenario is very rare; however, the community is working on a solution.
A number of locking-related decisions have to be made by the scheduler. The state of the runqueues would vary from the above example, depending on when the scheduling operation is performed on the runqueues. The above example has been simplified for this explanation.
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