(2014/08/08 23:28), Paul E. McKenney wrote: > On Fri, Aug 08, 2014 at 10:12:21AM -0400, Steven Rostedt wrote: >> On Fri, 8 Aug 2014 08:40:20 +0200 >> Peter Zijlstra <pet...@infradead.org> wrote: >> >>> On Thu, Aug 07, 2014 at 05:18:23PM -0400, Steven Rostedt wrote: >>>> On Thu, 7 Aug 2014 22:08:13 +0200 >>>> Peter Zijlstra <pet...@infradead.org> wrote: >>>> >>>>> OK, you've got to start over and start at the beginning, because I'm >>>>> really not understanding this.. >>>>> >>>>> What is a 'trampoline' and what are you going to use them for. >>>> >>>> Great question! :-) >>>> >>>> The trampoline is some code that is used to jump to and then jump >>>> someplace else. Currently, we use this for kprobes and ftrace. For >>>> ftrace we have the ftrace_caller trampoline, which is static. When >>>> booting, most functions in the kernel call the mcount code which >>>> simply returns without doing anything. This too is a "trampoline". At >>>> boot, we convert these calls to nops (as you already know). When we >>>> enable callbacks from functions, we convert those calls to call >>>> "ftrace_caller" which is a small assembly trampoline that will call >>>> some function that registered with ftrace. >>>> >>>> Now why do we need the call_rcu_task() routine? >>>> >>>> Right now, if you register multiple callbacks to ftrace, even if they >>>> are not tracing the same routine, ftrace has to change ftrace_caller to >>>> call another trampoline (in C), that does a loop of all ops registered >>>> with ftrace, and compares the function to the ops hash tables to see if >>>> the ops function should be called for that function. >>>> >>>> What we want to do is to create a dynamic trampoline that is a copy of >>>> the ftrace_caller code, but instead of calling this list trampoline, it >>>> calls the ops function directly. This way, each ops registered with >>>> ftrace can have its own custom trampoline that when called will only >>>> call the ops function and not have to iterate over a list. This only >>>> happens if the function being traced only has this one ops registered. >>>> For functions with multiple ops attached to it, we need to call the >>>> list anyway. But for the majority of the cases, this is not the case. >>>> >>>> The one caveat for this is, how do we free this custom trampoline when >>>> the ops is done with it? Especially for users of ftrace that >>>> dynamically create their own ops (like perf, and ftrace instances). >>>> >>>> We need to find a way to free it, but unfortunately, there's no way to >>>> know when it is safe to free it. There's no way to disable preemption >>>> or have some other notifier to let us know if a task has jumped to this >>>> trampoline and has been preempted (sleeping). The only safe way to know >>>> that no task is on the trampoline is to remove the calls to it, >>>> synchronize the CPUS (so the trampolines are not even in the caches), >>>> and then wait for all tasks to go through some quiescent state. This >>>> state happens to be either not running, in userspace, or when it >>>> voluntarily calls schedule. Because nothing that uses this trampoline >>>> should do that, and if the task voluntarily calls schedule, we know >>>> it's not on the trampoline. >>>> >>>> Make sense? >>> >>> Ok, so they're purely used in the function prologue/epilogue callchain. >> >> No, they are also used by optimized kprobes. This is why optimized >> kprobes depend on !CONFIG_PREEMPT. [ added Masami to the discussion ]. >> >> Which reminds me. On !CONFIG_PREEMPT, call_rcu_task() should be >> equivalent to call_rcu_sched(). > > Almost. One difference is that call_rcu_sched() won't wait for > idle-task execution. So presumably you are currently prohibited from > putting kprobes in idle tasks.
No need to prohibit all kprobes, just prohibit optimizing if the kprobe is in the idle context (if I can detect it). Since I've already replaced text-area based __kprobes with list-based NOKPROBE_SYMOBOL in core kernel, I think it is an option to add NOOPTPROBE_SYMBOL for that purpose. Thank you, > Oleg slipped this one past me, and for more than a full hour, > (https://lkml.org/lkml/2014/8/2/18), but this time I remembered. ;-) > > Thanx, Paul -- 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/
