Messages in this thread | | | Date | Thu, 03 Jul 2014 16:07:50 -0400 | From | Steven Rostedt <> | Subject | [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines |
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[ NOT READY FOR INCLUSION! ]
Note, this is based off of my remove ftrace_start/stop() patch set.
I've been wanting to do this for years, and just never gotten around to it. But with all this talk of kpatch and kgraft live kernel patching using the ftrace infrastructure, it seems like a good time to do it.
The way the function callback mechanism works in ftrace is that if there's only one function callback registered, it will set the mcount/fentry trampoline to call that function directly. But as soon as you register another callback, the mcount trampoline calls a loop function that iterates over all the registered callbacks (ftrace_ops) checking their hash tables to see if the called function matches the ops before calling its callback. This happens even if the two registered functions are not even tracing the same function!
This really sucks if you are tracing all functions, and then add a kprobe or perf event that traces a single function. That will cause all the other functions being traced to perform the loop test.
Ideally, if only a single callback (ftrace_ops) is registered to a function, than that function should call a trampoline that will only call that one callback without doing any other tests.
This patch set adds this functionality to x86_64. If a callback is registered to a function and there's no other callback registered to that function that ftrace_ops will get its own trampoline allocated for it that will call the function directly.
Note, for dynamically allocated ftrace_ops (kprobes, perf, instance directory function tracing), the dynamic trampoline will only be created if CONFIG_PREEMPT is not set. That's because, until Paul finishes his rcu_call_task() code, there's no safe way to know if a task was preempted while on the trampoline and is waiting to run on it some more.
I need to write up a bunch of tests for this code, but currently it works on the few tests I did manually. I didn't even run this code yet under my full test suite, so it may very well have bugs in it that might be easily triggered. But I wanted to get the code out for review to see if anyone has any other idea to help enhance this feature.
If you want a git repo to play with this, you can get it from below. That repo will rebase often, so do not build against it.
Enjoy,
-- Steve
git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git rfc/trampoline
Head SHA1: 4d781e010842a56f8e7c1bbe309e38075c277c45
Steven Rostedt (Red Hat) (3): ftrace/x86: Add dynamic allocated trampoline for ftrace_ops ftrace/x86: Show trampoline call function in enabled_functions ftrace/x86: Allow !CONFIG_PREEMPT dynamic ops to use allocated trampolines
---- arch/x86/kernel/ftrace.c | 240 ++++++++++++++++++++++++++++++++++++++++++-- arch/x86/kernel/mcount_64.S | 26 ++++- include/linux/ftrace.h | 8 ++ kernel/trace/ftrace.c | 86 +++++++++++++++- 4 files changed, 344 insertions(+), 16 deletions(-)
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