How do you test and verify interrupt latency requirements and specifications?

To start testing and verifying interrupt latency, the first thing I do is define the exact requirements for the system. This means I figure out what kind of response time the application actually needs—whether it’s super-fast, like microseconds for critical tasks, or a bit slower, like milliseconds for less time-sensitive tasks. I also set a maximum latency, which is the longest delay that’s acceptable, and an average latency for day-to-day performance. These requirements usually depend on how fast the processor is, how heavy the system’s workload is, and how the interrupts are prioritized.

Scheduling policy (at system level) has to documented which includes the number of interrupts, priorities, nested interrupts, allowed time consumption for each task execution. We can't decide the interrupt latency for specific interrupt. It is to be analyzed at system level.

After that, I need to choose the right method to measure the latency. There are two main options: hardware tools like oscilloscopes, which are really accurate, or software-based timers if I need a more lightweight solution. Personally, I lean toward hardware tools when I want precision, but software timers can also do the job if I’m just getting a rough idea.

My choice is debugger based profiling systems which clearly takes snapshots/continuous runtime snaps when the system is running at full load. This kind of tools are cost expensive, but worth to have it. 1. Analyze the result. 2. List out the possible improvements. 3. See the results. REPEAT.

Once I’ve gathered the data, I analyze the results. This is where I check if the system is meeting the expected latency limits. If there are any unexpected delays or spikes in response times, I dig deeper to figure out what’s causing them. It’s all about spotting patterns and making sure the system isn’t taking too long to respond when it shouldn’t.

Sometimes, it might be better to avoid using interrupts. or ask the question why do I need it? and do i really need it? Instead, you can use a timing arch with polling or if the design permit leverage the use of DMA to reduce the need for too much context switching. and stop using interrupt locking unnecessarily always find a way to avoid it. It's important to know how the hardware works, some interrupts happen only when a HW buffer fills up to a certain level. ex SPI FIFO buffer. this can help make interruptions happen less often (more efficient) The key is to really to understand the hardware and the kind of data you're dealing with.

To reduce interrupt latency, I try to keep the code handling interrupts as fast and simple as possible. I avoid long critical sections where interrupts are disabled and make sure that the interrupt service routines run quickly without unnecessary delays. It’s like fine-tuning a machine to make sure nothing slows it down.

Creating Real-world scenarios might not help to improve the interrupt latency. We have to inject test cases which creates war between interrupts, because that's what we are dealing. That would un-earth the gaps in scheduling policies. With that as evidence, change the scheduling policies and run the test cases.

I validate the system by running it through real-world scenarios and stress tests. This helps me confirm that the system can handle the defined latencies, even when things get tough or unpredictable. I test for worst-case situations to make sure everything stays within the limits I set at the beginning. Mostly I use python to validate all the software system which I wrote.

I always consider external factors that might impact latency, like hardware setups or other processes running alongside the system. It’s important to have a backup plan in case the system doesn’t meet the requirements, so I can make adjustments if needed.