Hi Ingo and Steven, Ingo Molnar wrote: > From: Steven Rostedt <[EMAIL PROTECTED]> > > This patch adds a feature to dynamically replace the ftrace code > with the jmps to allow a kernel with ftrace configured to run > as fast as it can without it configured. > > The way this works, is on bootup, a ftrace function is registered > to record the instruction pointer of all places that call the > function. > > Later, a kthread is awoken once a second that performs a stop_machine, > and replaces all the code that was called with a jmp over the call > to ftrace. It only replaces what was found the previous time. > > e.g. > > call ftrace /* 5 bytes */ > > is replaced with > > jmp 3f /* jmp is 2 bytes and we jump 3 forward */ > 3: > > When we want to enable ftrace for function tracing, the IP recording > is removed, and stop_machine is called again to replace all the locations > of that were recorded back to the call of ftrace. When it is disabled, > we replace the code back to the jmp. > > Allocation is done by the kthread. If the ftrace recording function is > called, and we don't have any record slots available, then we simply > skip that call. Once a second a new page (if needed) is allocated for > recording new ftrace function calls. A large batch is allocated at > boot up to get most of the calls there. > > Because we do this via stop_machine, we don't have to worry about another > CPU executing a ftrace call as we modify it. But we do need to worry > about NMI's so all functions that might be called via nmi must be > annotated with notrace_nmi. When this code is configured in, the NMI code > will not call notrace.
I'd like to suggest that you can use djprobe-like solution here to eliminate stop_machine. The djprobe makes a bypass over that call instruction by using a kprobe, and after replacing the call instruction, you can safely remove the bypass. Here is an ols paper about djprobe. http://www.kernel.org/doc/ols/2007/ols2007v1-pages-189-200.pdf Thank you, -- Masami Hiramatsu Software Engineer Hitachi Computer Products (America) Inc. Software Solutions Division e-mail: [EMAIL PROTECTED] -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
