Kernel Korner - AEM: a Scalable and Native Event Mechanism for Linux
From the AEM perspective, an event is a system stimulus that initiates the creation of an execution agent, the event handler. Support is provided by the event_struct, which is a structure initialized during event registration that contains the context necessary to execute one event handler. Some of the main fields are address of a user-space event handler, constructors and destructors for the event handler and relationship with other events (list of events, child events and active events—see Figure 3).
There can be as many registered events per process as necessary. When an event is detected, we say it is activated, and the user-defined callback function soon is executed. Events are linked internally in the same order as they arrive into the system. It then is up to each handler constructor to manage the data correctly and keep the events serialized for each process without presuming any order of arrival.
Some process events are active and linked to an active events list. Upon activation, an event can create a process. These events are called cloners, and the relationships between these events and created processes are recorded internally. An event registered by the top process in Figure 3 has created two new processes below it. They remain attached to this event and keep their own list of events.
Event handlers are used during event registrations and must be implemented at the user level. They define their own fixed set of parameters in order to provide event data completion directly to the user-space process. This operation is done by event constructors and destructors executed right before and right after handlers are called. Event handlers are executed in the same context as the calling process. The mechanism is safe and re-entrant; the current flow of execution is saved and then restored to its state prior to the interruption.
A priority is associated with each event during registration that represents the speed at which an application is interested to receive notification. It is possible to register twice for the same event with two different priorities.
Other real-time notification mechanisms, such as the real-time extension of POSIX signals, do not consider priorities during the scheduling decision. This is important, because it allows a process receiving a high-priority event to be scheduled before other processes. In AEM, the occurrence of an event pushes the event handler to be executed depending on its priority. To some extent, an event handler is a process, because it has an execution context. Changing process priorities dynamically is a real issue when the rate of event arrival is high, because priorities are updated quickly at the same rate. We solved this problem by introducing a dynamic soft real-time value calculated using a composition of event priorities. This value influences the scheduling decision without affecting the Linux scheduler and brings soft real-time responsiveness to applications.
A job is a new kernel abstraction introduced to serve events before notifying processes. It is not a process, although both share the same conceptual idea of executable entity. One typical action performed by a job is to insert itself into a wait queue and stay there until something wakes it up. At that point, it quickly performs some useful work, such as checking for data validity or availability before activating the user event, and goes back to sleep. A job also guarantees that while it is accessing some resource, no other job can access it. Several jobs can be associated with one process, but there is only one job per event.
This abstraction layer between the kernel and the user process is necessary. Otherwise, it is difficult to ensure consistency in checking for data availability or agglomerating multiple occurrences of the same event when the handler is executed. If something goes wrong, the process wastes time handling the event in user space. Deciding whether to concatenate several notifications is event-specific and should be resolved before event activation.
A generic implementation of jobs would consider software interrupts in order to have a short latency between the time an event occurred and the time the process is notified. The goal is to execute on behalf of processes and provide the same capabilities as both an interrupt handler and a kernel thread, without dragging along a complete execution context.
Two types of jobs are implemented, periodic jobs and reactive jobs. Periodic jobs are executed at regular intervals, and reactive jobs are executed sporadically, upon reception of an event. Jobs are scheduled by their own off-line scheduler. According to the real-time theory of scheduling, both types of jobs could be managed by the same scheduler (see the Jeffay et al. paper in the on-line Resources). In our context, a job is a nonpreemptive task. By definition, jobs have no specific deadlines, although their execution time should be bounded implicitly because of their low-level nature. This assumption simplifies the implementation. The constraint in our case is for reactive jobs to be able to execute with a negligible time interval between two invocations so as to satisfy streaming transfer situations.
Our implementation of periodic and reactive jobs is different in both cases in order to obtain a better throughput in case of sporadic events. A job scheduler and a dispatcher handle periodic jobs, whereas reactive jobs change state themselves for performance reasons. Figure 4 describes the job state evolution and functions used to move from one state to another.
Once a job has activated the corresponding event, either a process is executed asynchronously or the current flow of execution of the user program is redirected somewhere else in the code. The user decides how to handle events at registration time.
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