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Fix argdist, trace, tplist to use the libbcc USDT support (#698)
* Allow argdist to enable USDT probes without a pid The current code would only pass the pid to the USDT class, thereby not allowing USDT probes to be enabled from the binary path only. If the probe doesn't have a semaphore, it can actually be enabled for all processes in a uniform fashion -- which is now supported. * Reintroduce USDT support into tplist To print USDT probe information, tplist needs an API to return the probe data, including the number of arguments and locations for each probe. This commit introduces this API, called bcc_usdt_foreach, and invokes it from the revised tplist implementation. Although the result is not 100% identical to the original tplist, which could also print the probe argument information, this is not strictly required for users of the argdist and trace tools, which is why it was omitted for now. * Fix trace.py tracepoint support Somehow, the import of the Perf class was omitted from tracepoint.py, which would cause failures when trace enables kernel tracepoints. * trace: Native bcc USDT support trace now works again by using the new bcc USDT support instead of the home-grown Python USDT parser. This required an additional change in the BPF Python API to allow multiple USDT context objects to be passed to the constructor in order to support multiple USDT probes in a single invocation of trace. Otherwise, the USDT-related code in trace was greatly simplified, and uses the `bpf_usdt_readarg` macros to obtain probe argument values. One minor inconvenience that was introduced in the bcc USDT API is that USDT probes with multiple locations that reside in a shared object *must* have a pid specified to enable, even if they don't have an associated semaphore. The reason is that the bcc USDT code figures out which location invoked the probe by inspecting `ctx->ip`, which, for shared objects, can only be determined when the specific process context is available to figure out where the shared object was loaded. This limitation did not previously exist, because instead of looking at `ctx->ip`, the Python USDT reader generated separate code for each probe location with an incrementing identifier. It's not a very big deal because it only means that some probes can't be enabled without specifying a process id, which is almost always desired anyway for USDT probes. argdist has not yet been retrofitted with support for multiple USDT probes, and needs to be updated in a separate commit. * argdist: Support multiple USDT probes argdist now supports multiple USDT probes, as it did before the transition to the native bcc USDT support. This requires aggregating the USDT objects from each probe and passing them together to the BPF constructor when the probes are initialized and attached. Also add a more descriptive exception message to the USDT class when it fails to enable a probe.
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