út 22. 7. 2025 v 0:35 odesílatel Steven Rostedt <rost...@goodmis.org> napsal: > > I'm curious to what is looked for for triggering an action. We can poll on > events and get woken when they are triggered. It may be possible to add > even more ways to wake a task waiting for something to happen. >
Threshold actions are triggered immediately after a sample over the set threshold is detected by rtla. For BPF mode, this happens almost right after the sample is processed in the BPF program and the scheduler gets to waking up rtla following a BPF ringbuffer write. There is only a short delay (up to tens of microseconds) because the BPF helper defers the wake-up into irq_work. For non-BPF mode, rtla periodically pulls samples from tracefs, when it does that, it also checks whether tracing has been turned off. If yes, that means there was a threshold overflow, and actions are triggered. Since the period for that is currently set to 1 second, the action might be delayed up to one second from the threshold occurring, That delay might be a problem if you need to collect a lot of data from a ringbuffer in the action, e.g. global Intel PT data collection for precise troubleshooting of difficult latencies. Of course, this is just an implementational limitation of the timerlat tracer. If timerlat had an event (like osnoise's "sample_threshold") triggered on threshold overflow and if it is possible to wait on it even without BPF, rtla could wait on that for both BPF and non-BPF mode instead of what it is currently doing. Tomas
