(2014/07/04 5:07), Steven Rostedt wrote: > [ 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.
Ah, I've thought that ftrace already had different trampoline for loop and single and replaced each mcount-call instruction to appropriate one. But this series actually does that, doesn't this? :) > 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. Hmm, if we can declare "this ftrace_ops is permanent"(like finalizing) then we can allocate trampoline for such dynamic one. Since the kprobes actually doesn't need to free (or unregister) ftrace_ops, I can use it. > 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. Yeah, I'll review it. Thank you, > > 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(-) > > > -- Masami HIRAMATSU Software Platform Research Dept. Linux Technology Research Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: masami.hiramatsu...@hitachi.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
