erts: Add internal docs for code loading and tracing

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+Non-blocking trace setting
+==========================
+
+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, trace 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.
+
+
+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.
+
+
+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 choosen 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 indentified by index of 0 and
+1. The global atomic variables `erts_active_bp_index` will determine
+which generation of breakpoints running code will use.
+
+### Atomicy 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.
+
+
+Adding a new Breakpoint
+-----------------------
+This is a simplified sequence describing what `trace_pattern` goes
+through when adding a new breakpoint.
+
+1. Seize exclusive code write 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 -4 from the first
+   instruction "func_info" 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 -4.
+
+7. Wait for thread progress.
+
+8. Commit the breadpoint 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 the new active part.
+
+11. Release code write permission and return from `trace_pattern`.
+
+
+The code write permission "lock" seized in step 1 is the same as used
+by code loading. This will ensure that only one process at a time can
+stage new trace settings but it will also prevent concurrent code
+loading 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.
+
+
+To Updating 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 write 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
+   "func_info" header at offset -4.
+
+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 breadpoints 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 write permission and return from `trace_pattern`.
+
+
+### 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 dont'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.
+
+
+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.
+
+
+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.