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Message-ID: <20230801204054.3884688e@rorschach.local.home>
Date: Tue, 1 Aug 2023 20:40:54 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
linux-trace-kernel@...r.kernel.org,
LKML <linux-kernel@...r.kernel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>,
bpf <bpf@...r.kernel.org>, Sven Schnelle <svens@...ux.ibm.com>,
Alexei Starovoitov <ast@...nel.org>,
Jiri Olsa <jolsa@...nel.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Alan Maguire <alan.maguire@...cle.com>,
Mark Rutland <mark.rutland@....com>,
Florent Revest <revest@...omium.org>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a
struct/union
On Wed, 2 Aug 2023 09:21:46 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> 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@kernel.org/
>
> 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).
And this still doesn't work on arm64.
Maybe we can add a ftrace_partial_regs(fregs) that returns a
partially filled pt_regs, and the caller that uses this obviously knows
its partial (as it's in the name). But this doesn't quite help out arm64
because unlike x86, struct ftrace_regs does not contain an address
compatibility with pt_regs fields. It would need to do a copy.
ftrace_partial_regs(fregs, ®s) ?
-- Steve
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