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

17.1.4. Latency

17.1.4. Latency

Real-time processes are often associated with a physical event, such as an interrupt arriving from a peripheral device. Figure 17-1 illustrates the latency components in a Linux system. Latency measurement begins upon receipt of the interrupt we want to process. This is indicated by time t0 in Figure 17-1. Sometime later, the interrupt is taken and control is passed to the Interrupt Service Routine (ISR). This is indicated by time t1. This interrupt latency is almost entirely dictated by the maximum interrupt off time[115] the time spent in a thread of execution that has hardware interrupts disabled.

Figure 17-1. Latency components


It is considered good design practice to minimize the processing done in the actual interrupt service routine. Indeed, this execution context is limited in capability (for example, an ISR cannot call a blocking function, one that might sleep), so it is desirable to simply service the hardware device and leave the data processing to a Linux bottom half,[116] also called softIRQs.

When the ISR/bottom half has finished its processing, the usual case is to wake up a user space process that is waiting for the data. This is indicated by time t2 in Figure 17-1. At some point in time later, the real-time process is selected by the scheduler to run and is given the CPU. This is indicated by time t3 in Figure 17-1. Scheduling latency is affected primarily by the number of processes waiting for the CPU and the priorities among them. Setting the Real Time attribute on a process gives it higher priority over normal Linux processes and allows it to be the next process selected to run, assuming that it is the highest priority real-time process waiting for the CPU. The highest-priority real-time process that is ready to run (not blocked on I/O) will always run. You'll see how to set this attribute shortly.

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