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tracing: only prepare tracepoint args if attached
Before this commit, we would always prepare tracepoint arguments regardless of the tracepoint being used or not. While we already made sure not to include expensive arguments in our tracepoints, this commit introduces gating to make sure the arguments are only prepared if the tracepoints are actually used. This is a win-win improvement to our tracing framework. For users not interested in tracing, the overhead is reduced to a cheap 'greater than 0' compare. As the semaphore-gating technique used here is available in bpftrace, bcc, and libbpf, users interested in tracing don't have to change their tracing scripts while profiting from potential future tracepoints passing slightly more expensive arguments. In the context of bitcoin#26531's mempool tracepoints, one example could be passing serialized transactions for RBF replacements. Under the hood, the semaphore-gating works by placing a 2-byte semaphore in the '.probes' ELF section. The address of the semaphore is contained in the ELF note providing the tracepoint information (`readelf -n ./src/bitcoind | grep NT_STAPSDT`). Tracing toolkits like bpftrace, bcc, and libbpf increase the semaphore at the address upon attaching to the tracepoint. We only prepare the arguments and reach the tracepoint if the semaphore is greater than zero. The semaphore is decreased when detaching from the tracepoint. The implementation of the TRACEPOINT_ACTIVE macro is flexible enough that it can be adapted for macOS dtrace tracing (bitcoin#25541). With dtrace, [CONTEXT]_[EVENT]_ENABLED() can be used to gate the tracepoints. While the TRACEPOINT0(context, event) macro without arguments doesn't need a check if it's active or not (no arguments can be prepared and passed), it does require a TRACEPOINT_SEMAPHORE() due to _SDT_HAS_SEMAPHORES requiring all tracepoints to have a semaphore. As the check is cheap, a check is added to not cause problems with the semaphore being unused. This also extends the "Adding a new tracepoint" documentation to include information about the semaphores and updated step-by-step instructions on how to add a new tracepoint.
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