On Tue, 1 Aug 2023 20:40:54 -0400 Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > On Wed, 2 Aug 2023 09:21:46 +0900 > Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> wrote: > > > > Then use kprobes. When I asked Masami what the difference between fprobes > > > and kprobes was, he told me that it would be that it would no longer rely > > > on the slower FTRACE_WITH_REGS. But currently, it still does. > > > > kprobes needs to keep using pt_regs because software-breakpoint exception > > handler gets that. And fprobe is used for bpf multi-kprobe interface, > > but I think it can be optional. > > > > So until user-land tool supports the ftrace_regs, you can just disable > > using fprobes if CONFIG_DYNAMIC_FTRACE_WITH_REGS=n > > I'm confused. I asked about the difference between kprobes on ftrace > and fprobes, and you said it was to get rid of the requirement of > FTRACE_WITH_REGS. > > https://lore.kernel.org/all/20230120205535.98998636329ca4d5f8325bc3@xxxxxxxxxx/ Yes, it is for enabling fprobe (and fprobe-event) on more architectures. I don't think it's possible to change everything at once. So, it will be changed step by step. At the first step, I will replace pt_regs with ftrace_regs, and make bpf_trace.c and fprobe_event depends on FTRACE_WITH_REGS. At this point, we can split the problem into two, how to move bpf on ftrace_regs and how to move fprobe-event on ftrace_regs. fprobe-event change is not hard because it is closing in the kernel and I can do it. But for BPF, I need to ask BPF user-land tools to support ftrace_regs. > > > > > Then you can safely use > > > > struct pt_regs *regs = ftrace_get_regs(fregs); > > > > I think we can just replace the CONFIG_FPROBE ifdefs with > > CONFIG_DYNAMIC_FTRACE_WITH_REGS in kernel/trace/bpf_trace.c > > And that will be the first version of using ftrace_regs in fprobe. > > But it is still slow. The FTRACE_WITH_REGS gives us the full pt_regs > and saves all registers including flags, which is a very slow operation > (and noticeable in profilers). Yes, to solve this part, we need to work with BPF user-land people. I guess the BPF is accessing registers from pt_regs with fixed offset which is calculated from pt_regs layout in the user-space. > > And this still doesn't work on arm64. Yes, and this makes more motivation to move on ftrace_regs. Thank you, -- Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
