Messages in this thread | | | Date | Thu, 21 Apr 2022 16:14:13 +0100 | From | Mark Rutland <> | Subject | Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines |
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On Thu, Apr 21, 2022 at 10:06:39AM -0400, Steven Rostedt wrote: > 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.
Thanks for the, explanation, btw!
> 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).
Ok, so that's when messing around with bpf or kprobes, and not generally when using plain old ftrace functionality under /sys/kernel/tracing/ (unless that's concurrent with one of the former, as per your other reply) ?
> 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.
Sure; I can see how that can be quite expensive.
What I'm trying to figure out is who this matters to and when, since the implementation is going to come with a bunch of subtle/fractal complexities, and likely a substantial overhead too when enabling or disabling tracing of a patch-site. I'd like to understand the trade-offs better.
> 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.
I see -- so the dynamic trampoline is just to get the ops? Or is that doing additional things?
There might be a middle-ground here where we patch the ftrace_ops pointer into a literal pool at the patch-site, which would allow us to handle this atomically, and would avoid the issues with out-of-range trampolines.
Thanks, Mark.
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