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| -rw-r--r-- | tapsets/dynamic_cg/dynamic_cg.txt | 64 |
1 files changed, 64 insertions, 0 deletions
diff --git a/tapsets/dynamic_cg/dynamic_cg.txt b/tapsets/dynamic_cg/dynamic_cg.txt new file mode 100644 index 00000000..35de29f1 --- /dev/null +++ b/tapsets/dynamic_cg/dynamic_cg.txt @@ -0,0 +1,64 @@ +* Application name: Dynamic Callgraph +* Contact: William Cohen, wochen@redhat.com + +* Motivation: + +Dynamic Callgraph would provide information to allow developers to see +what other functions a function is calling. This could show that some +unexpected functions are getting called. DTrace has a instrumentation +provider that generates a trace of the functions called and returned. + +* Background: + +There have been times that people in Red Hat support have narrowed a +problem to a specific function and the functions it calls. Rather +than instrumenting the function's children by hand, a tapset that +provides a dynamic callgraph would allow quicker determination of the +things called. There are cases in the kernel code where determining +the function being called cannot be determined statically, +e.g. function to call is stored in a data structure. + +* Target software: + +Ideally both kernel and user space, but kernel space only would +be sufficient for many cases. + +* Type of description: tapset and scripting command + tapset to provide to support to capture call return information + scripting commands to turn on and off the capture + +* Interesting probe points: + +* Interesting values: + +* Dependencies: +- P6/x86-64 processors have the debug hardware to trap control flow chgs. +- Need to have the kernel maintain the debug hardware on a per process basis. + The DebugCtlMSR is not currently stored in the context + (only debug registers 0, 1, 2, 3, 6, and 7 are virtualized) + +* Restrictions: + May be difficult to implement on ppc: returns may look like regular jumps + and trapping on all branches could cause problems with atomic operations + Won't work on pre p6 x86 processors + Won't provide data for inlined funcions + +* Data collection: + Track whether the instruction was a call or a return and the target address. + +* Data presentation: + -processing address in userspace to convert addresses into function names + -trace showing calls and returns + -maybe further post process to build dynamic callgraph + determine that a function is being called way too often + +* Competition: + DTrace already implements tracing of function calls and returns. + +* Cross-references: + +* Associated files: + + $dynamic_call_graph = 1; // turn on tracing of calls for thread + $dynamic_call_graph = 0; // turn off tracing of calls for thread + |