From: dale.hamel@... Date: 2019-07-28T13:15:44+00:00 Subject: [ruby-core:93957] [Ruby master Feature#16027] Update Ruby's dtrace / USDT API to match what is exposed via the TracePoint API Issue #16027 has been updated by dalehamel (Dale Hamel). > As far as I understand, target provides just an internal filtering, so I wonder how it involves external USDT API. The external USDT API is "always on" or "always off", it must be enabled for all events of a type or not used at all. I don't believe that the newly added filtering will affect it in any way. > What data will the "list of simple types" match against, the arguments to TracePoint#fire? Yes, this would just be used if the user specifies a block handler for the Tracepoint, so it must be the user that decides if data will be sent to the Tracepoint (as I cannot think if there is any way to do this dynamically). Unlike the built-in USDT tracepoints, these enabled tracepoints I think would need to be more dynamic. They could also be standardized, so that they use the same types by default as the dtrace events of the type being traced. For example, for the call event type, the USDT api via dtrace (https://github.com/ruby/ruby/blob/master/doc/dtrace_probes.rdoc) is: > ruby:::method-entry(classname, methodname, filename, lineno); > This probe is fired just before a method is entered. > classname name of the class (a string) > methodname name of the method about to be executed (a string) > filename the file name where the method is _being called_ (a string) > lineno the line number where the method is _being called_ (an int) So then perhaps for tracepoints enabled on the :call event type the default value for this should be [String, String, String, Integer]. If a user wants to change what they fire instead, they can override this with a different list of basic types. This keeps it the same as the existing USDT API, while offering the possibility for extension. > You seem to hope JIT can hook your feature easily, but I believe that's not true. For example, JIT often fallbacks to VM interpretation when some unexpected things like a method redefinition or TracePoint enablement happen. If we implemented your feature using JIT, the introspection would be randomly disabled and sporadic. We should use right tools for the right place. JIT is for optimization, not for debugging. Thank you for pointing this out. Yes, I don't have a lot of confidence in this idea either, but I thought I would still mention it as a potential implementation possibility. If this feature is accepted, I think it would make more sense to do ELF/DOF generation by Ruby instead. I mentioned this mostly because I thought there may be a chance the ruby JIT faculties could simplify implementation, but I would not count on this. Thank you for your replies and feedback nobu and k0kubun, I hope my response addresses your concerns and clarify the description for you. ---------------------------------------- Feature #16027: Update Ruby's dtrace / USDT API to match what is exposed via the TracePoint API https://bugs.ruby-lang.org/issues/16027#change-80147 * Author: dalehamel (Dale Hamel) * Status: Open * Priority: Normal * Assignee: * Target version: ---------------------------------------- # Abstract I propose that Ruby's "dtrace" support be extended to match what is available in the TracePoint API, as was the case until feature [Feature #15289] landed. # Background I will refer to Ruby's "dtrace" bindings as USDT bindings for simplicity, as this is the typo of dtrace probe that they support. Prior to [Feature #15289] being merged, Ruby's tracepoint API was able to trace only 'all' instances of a type of event. Ruby added support for tracing ruby with dtrace, and so Ruby's USDT Ruby TracePoint API were "in sync". Once the Ruby TracePoint API recently added the ability to do filtered tracing in [Feature #15289], it added new functionality but brought the TracePoint and USDT API out of sync. Currently the TracePoint API is ahead of the USDT API, which presents the problem. There is valuable debug information available, but we do not have a way to access it with dtrace instrumentation. Additionally, the recent release of bpftrace adds support for USDT tracing on linux, which makes this a valuable opportunity to be able to use Ruby's TracePoint API in an efficient and targeted way for production tracing. To achieve this, we must synchronize the features of the USDT and TracePoint API. What is currently lacking is the ability to do filtered, selective tracing as the `TracePoint#enable` call now supports as per [prelude.rb#L141](https://github.com/ruby/ruby/blob/master/prelude.rb#L141) # Proposal When enabling a TracePoint, users can specify a flag: `usdt: [LIST_OF_SIMPLE_TYPES]`, which will trigger Ruby to also enable the USDT API for when it enables TracePoints. Within the TracePoint block, users can call `tp.fire` to send USDT data. So the new default API is: ```ruby trace.enable(target: nil, target_line: nil, target_thread: nil: usdt: nil) ``` And the usage might look like: ```ruby trace.enable(target: method(:foo), target_line: 5, usdt: [Integer, String]) do |tp| tp.fire(tp.lineno, "Any String I want to send") end ``` The types specified must be simple types such as `Integer` or `String`, given by their names as constants. When data to the tracepoint, the types must match. If they don't, the tracer won't be able to interpret them properly, but nothing should crash. # Details I propose that Ruby optionally generate ELF (Linux) or DOF (Darwin) annotations for TracePoint targets when they are enabled. As ruby is a dynamic language, it cannot do this natively (yet) though Ruby JIT may make this easier, but for now it is not suitable for production use. To get around this, Ruby can either generate the DOF or ELF stub shared library itself, for example it may do one per class, treating the class as the "provider" for the USDT API, and the methods as tracepoints. This is the approach used by [libusdt](https://github.com/chrisa/libusdt), which generates DOF usable on Darwin, BSD, and other platforms, and [libstapsdt](https://github.com/sthima/libstapsdt), which generates ELF stubs for use on linux. When a tracepoint is triggered, the user may be able to call a new API `TracePoint#fire`, to send data to the Kernel via the USDT API, using the generated ELF stub as a bridge, giving the kernel an address to target in order to receive this data. Upon enabling a tracepoint, we can either generate these stubs internally, or by linking to an external library that must be enabled at configure time (without this, USDT tracing wouldn't be enabled at all). It may be possible to use the existing bridge that is used by ruby jit, or have an experimental flag such as `--usdt` that enables support for generating these stubs. It may be more consistent with the future Ruby JIT to do this, or else Ruby can generate these stubs by its own native code, but this will require a sort of merging of libusdt and libstapsdt. This would add a dependency to the libelf development header, but that is probably not a problem on Linux platforms. I would suggest the first approach, if this feature is accepted, would be to try and implement the ELF / DOF generation directly in Ruby. What libstapsdt and libusdt do isn't that complex and could be done in its own C file that probably wouldn't be too large. Failing that approach, it may be worth investigating the Ruby JIT code to see if a compiler can generate these stubs for us easily. This approach would be to have ruby generate C code that results in the necessary DOF/ELF annotations, and have the compiler pipeline used by ruby JIT to generate the file. This couples the feature to ruby jit though. # Usecase This feature would be used by dtrace / bpftrace users to debug ruby applications. It may be possible for other platforms to benefit from this too, but I think the main use case is for Linux system administrators and developers to use external debuggers (dtrace/bpftrace) to introspect Ruby's behavior. # Discussion ## Pros: * Syncs the Ruby TracePoint and USDT API * Allows for much more dynamic and targeted USDT tracing * Can help to find problems in both development and production * Can be used for performance and error analysis * Is better than printing, as emitting/collecting data is only done while a "debugger is attached" ## Cons: * Complexity introduced, in order to generate the ELF/DOF stub files * Not easily ported to other platforms * Isn't fully consistent with the current dtrace functionality of Ruby, which is built-in to the VM # Limitation This will only work on *Nix platforms, and probably just on Linux to start, as that is where most of the benefits are. If the Ruby JIT approach is preferred or much simpler, then that functionality will be tied to the Ruby JIT functionality. # See also * https://bpf.sh/usdt-report-doc/index.html a document describing my experimental gem ruby-static-tracing, which prototypes this functionality outside of the RubyVM * https://bpf.sh/production-breakpoints-doc/index.html a work-in-progress on adding more dynamic method and line based USDT tracing to ruby, built atop ruby-static-tracing now using the ruby tracepoint API. -- https://bugs.ruby-lang.org/ Unsubscribe: