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

[Jiri Kosina]

On Thu, 3 Jul 2014, 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.
[ ... snip ... ]

For the record -- I just did a quick test, and kGraft works nicely on top 
of this patchset.

Will be looking into the patches more closely later this week.

-- 
Jiri Kosina
SUSE Labs
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/