# 7 Tracing

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7.1 Implementation

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Call, return, and exception tracing

Tracing is implemented by setting breakpoints in the traced functions, and sending the appropriate trace messages when they're hit.

- Call trace messages are sent immediately.
- Return tracing pushes a frame to the stack which returns to an instruction that sends a trace message when encountered.
- Exception tracing also pushes a frame to the stack, but will only send a trace message if we encounter it in stack scanning while throwing an exception.

This means that one must be careful not to use return or exception tracing on functions that never return, as each call pushes a frame that will never be removed.

Another limitation is that since the breakpoint is in the callee and not the caller, we're limited to the information we have on function ingress. This means that we can't actually tell who called us: since we're limited to inspecting the stack we can only say where we're going to return to, which is not quite the same thing.

As an illustration, when the caller option is enabled all trace messages from bar/1 will say that they were called from foo/0, even though it went through a bunch of other functions on the way:

    foo() ->
        lots(),
        ok.

    lots() ->
        'of'().

    'of'() ->
        indirections().

    indirections() ->
        bar(10).

    bar(0) ->
        done;
    bar(N) ->
        bar(N - 1).

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Export tracing

In the interpreter, breakpoints are set inside the code trampoline for export entries, and their address vector is updated to point to them. This way, only remote calls will hit the breakpoint while local calls to the same function are left alone, but it otherwise acts the same way as local breakpoints.

Things get a bit more involved in the JIT. See BeamAsm.md for more details.

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7.2 Setting breakpoints

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Introduction

Before OTP R16 when trace settings were changed by erlang:trace_pattern, all other execution in the VM were halted while the trace operation was carried out in single threaded mode. Similar to code loading, this can impose a severe problem for availability that grows with the number of cores.

In OTP R16, breakpoints are set in the code without blocking the VM. Erlang processes may continue executing undisturbed in parallel during the entire operation. The same base technique is used as for code loading. A staging area of breakpoints is prepared and then made active with a single atomic operation.

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Redesign of Breakpoint Wheel

To make it easier to manage breakpoints without single threaded mode a redesign of the breakpoint mechanism has been made. The old "breakpoint wheel" data structure was a circular double-linked list of breakpoints for each instrumented function. It was invented before the SMP emulator. To support it in the SMP emulator, is was essentially expanded to one breakpoint wheel per scheduler. As more breakpoint types have been added, the implementation have become messy and hard to understand and maintain.

In the new design the old wheel was dropped and instead replaced by one struct (GenericBp) to hold the data for all types of breakpoints for each instrumented function. A bit-flag field is used to indicate what different type of break actions that are enabled.

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Same Same but Different

Even though trace_pattern use the same technique as the non-blocking code loading with replicated generations of data structures and an atomic switch, the implementations are quite separate from each other. One initial idea was to use the existing mechanism of code loading to do a dummy load operation that would make a copy of the affected modules. That copy could then be instrumented with breakpoints before making it reachable with the same atomic switch as done for code loading. This approach seems straight forward but has a number of shortcomings, one being the large memory footprint when many modules are instrumented. Another problem is how execution will reach the new instrumented code. Normally loaded code can only be reached through external functions calls. Trace settings must be activated instantaneously without the need of external function calls.

The chosen solution is instead for tracing to use the technique of replication applied on the data structures for breakpoints. Two generations of breakpoints are kept and identified by index of 0 and 1. The global atomic variables erts_active_bp_index will determine which generation of breakpoints running code will use.

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Atomicity Without Atomic Operations

Not using the code loading generations (or any other code duplication) means that trace_pattern must at some point write to the active beam code in order for running processes to reach the staged breakpoints structures. This can be done with one single atomic write operation per instrumented function. The beam instruction words are however read with normal memory loads and not through the atomic API. The only guarantee we need is that the written instruction word is seen as atomic. Either fully written or not at all. This is true for word aligned write operation on all hardware architectures we use.

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Adding a new Breakpoint

This is a simplified sequence describing what trace_pattern goes through when adding a new breakpoint.

1. Seize exclusive code modification permission (suspend process until we get it).

2. Allocate breakpoint structure GenericBp including both generations. Set the active part as disabled with a zeroed flagfield. Save the original instruction word in the breakpoint.

3. Write a pointer to the breakpoint at offset -sizeof(UWord) from the first instruction ErtsFuncInfo header.

4. Set the staging part of the breakpoint as enabled with specified breakpoint data.

5. Wait for thread progress.

6. Write a op_i_generic_breakpoint as the first instruction for the function. This instruction will execute the breakpoint that it finds at offset -sizeof(UWord).

7. Wait for thread progress.

8. Commit the breakpoint by switching erts_active_bp_index.

9. Wait for thread progress.

10. Prepare for next call to trace_pattern by updating the new staging part (the old active) of the breakpoint to be identic to the new active part.

11. Release code modification permission and return from trace_pattern.

The code modification permission "lock" seized in step 1 is also taken by code loading. This ensures that only one process at a time can stage new trace settings, and also prevents concurrent codeloading and make sure we see a consistent view of the beam code during the entire sequence.

Between step 6 and 8, runninng processes might execute the written op_i_generic_breakpoint instruction. They will get the breakpoint structure written in step 3, read erts_active_bp_index and execute the corresponding part of the breakpoint. Before the switch in step 8 becomes visible they will however execute the disabled part of the breakpoint structure and do nothing other than executing the saved original instruction.

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To Update and Remove Breakpoints

The above sequence did only describe adding a new breakpoint. We do basically the same sequence to update the settings of an existing breakpoint except step 2,3 and 6 can be skipped as it has already been done.

To remove a breakpoint some more steps are needed. The idea is to first stage the breakpoint as disabled, do the switch, wait for thread progress and then remove the disabled breakpoint by restoring the original beam instruction.

Here is a more complete sequence that contains both adding, updating and removing breakpoints.

1. Seize exclusive code modification permission (suspend process until we get it).

2. Allocate new breakpoint structures with a disabled active part and the original beam instruction. Write a pointer to the breakpoint in ErtsFuncInfo header at offset -sizeof(UWord).

3. Update the staging part of all affected breakpoints. Disable breakpoints that are to be removed.

4. Wait for thread progress.

5. Write a op_i_generic_breakpoint as the first instruction for all functions with new breakpoints.

6. Wait for thread progress.

7. Commit all staged breakpoints by switching erts_active_bp_index.

8. Wait for thread progress.

9. Restore original beam instruction for disabled breakpoints.

10. Wait for thread progress.

11. Prepare for next call to trace_pattern by updating the new staging area (the old active) for all enabled breakpoints.

12. Deallocate disabled breakpoint structures.

13. Release code modification permission and return from trace_pattern.

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All that Waiting for Thread Progress

There are four rounds of waiting for thread progress in the above sequence. In the code loading sequence we sacrificed memory overhead of three generations to avoid a second round of thread progress. The latency of trace_pattern should not be such a big problem for however, as it is normally not called in a rapid sequence.

The waiting in step 4 is to make sure all threads will see an updated view of the breakpoint structures once they become reachable through the op_i_generic_breakpoint instruction written in step 5.

The waiting in step 6 is to make the activation of the new trace settings "as atomic as possible". Different cores might see the new value of erts_active_bp_index at different times as it is read without any memory barrier. But this is the best we can do without more expensive thread synchronization.

The waiting in step 8 is to make sure we don't restore the original bream instructions for disabled breakpoints until we know that no thread is still accessing the old enabled part of a disabled breakpoint.

The waiting in step 10 is to make sure no lingering thread is still accessing disabled breakpoint structures to be deallocated in step 12.

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Global Tracing

Call tracing with global option only affects external function calls. This was earlier handled by inserting a special trace instruction in export entries without the use of breakpoints. With the new non-blocking tracing we want to avoid special handling for global tracing and make use of the staging and atomic switching within the breakpoint mechanism. The solution was to create the same type of breakpoint structure for a global call trace. The difference to local tracing is that we insert the op_i_generic_breakpoint instruction (with its pointer at offset -4) in the export entry rather than in the code.

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Future work

We still go to single threaded mode when new code is loaded for a module that is traced, or when loading code when there is a default trace pattern set. That is not impossible to fix, but that requires much closer cooperation between tracing BIFs and the loader BIFs.