Messages in this thread | | | Date | Thu, 21 Apr 2022 10:06:39 -0400 | From | Steven Rostedt <> | Subject | Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines |
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On Thu, 21 Apr 2022 14:10:04 +0100 Mark Rutland <mark.rutland@arm.com> wrote:
> On Wed, Mar 16, 2022 at 06:01:31PM +0800, Wang ShaoBo wrote: > > From: Cheng Jian <cj.chengjian@huawei.com> > > > > When tracing multiple functions customly, a list function is called > > in ftrace_(regs)_caller, which makes all the other traced functions > > recheck the hash of the ftrace_ops when tracing happend, apparently > > it is inefficient. > > ... and when does that actually matter? Who does this and why?
I don't think it was explained properly. What dynamically allocated trampolines give you is this.
Let's say you have 10 ftrace_ops registered (with bpf and kprobes this can be quite common). But each of these ftrace_ops traces a function (or functions) that are not being traced by the other ftrace_ops. That is, each ftrace_ops has its own unique function(s) that they are tracing. One could be tracing schedule, the other could be tracing ksoftirqd_should_run (whatever).
Without this change, because the arch does not support dynamically allocated trampolines, it means that all these ftrace_ops will be registered to the same trampoline. That means, for every function that is traced, it will loop through all 10 of theses ftrace_ops and check their hashes to see if their callback should be called or not.
With dynamically allocated trampolines, each ftrace_ops will have their own trampoline, and that trampoline will be called directly if the function is only being traced by the one ftrace_ops. This is much more efficient.
If a function is traced by more than one ftrace_ops, then it falls back to the loop.
-- Steve
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