Re: [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines

[Steven Rostedt]

On Fri, 04 Jul 2014 22:20:12 +0900
Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> wrote:

> (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? :)

Well, I guess the answer to that is what do you consider the
trampoline? I'm currently considering it to be the assembly code that
the mcount/fentry call jumps to. We only have two trampolines (three if
you count the function graph code that will be called directly come
3.17). Those two are the normal ftrace_caller and the
ftrace_regs_caller. Now what they call can be different. When only a
single ftrace_ops is registered, they call the ftrace_ops->func
directly. If there are more than one ftrace_ops registered, then they
call the loop function directly.


> 
> > 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.

Yeah, if we add a ftrace_ops PERMANENT flag, then we could allow them
too.

> 
> 
> > 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.

Thanks!

-- Steve


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