Messages in this thread | | | Subject | Re: [RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines | From | "Wangshaobo (bobo)" <> | Date | Wed, 27 Apr 2022 16:54:03 +0800 |
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在 2022/4/21 22:06, Steven Rostedt 写道: > 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 > .
yes, this explanation is easier to understand, I will update commit description according to this.
-- Wang ShaoBo
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