linux-kernel - Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a struct/union

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Message-Id: <20230802225634.f520080cd9de759d687a2b0a@kernel.org>
Date:   Wed, 2 Aug 2023 22:56:34 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.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 Tue, 1 Aug 2023 20:40:54 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> 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/

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@...nel.org>