linux-kernel - Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL

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Message-ID: <CANk7y0jpKb8XUycKxHbsrJGEWcTvVDYFjM+A2VUe0JeZ2Y-Zbw@mail.gmail.com>
Date: Fri, 8 Mar 2024 15:22:17 +0100
From: Puranjay Mohan <puranjay12@...il.com>
To: Björn Töpel <bjorn@...nel.org>
Cc: Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Guo Ren <guoren@...nel.org>, 
	Ley Foon Tan <leyfoon.tan@...rfivetech.com>, Deepak Gupta <debug@...osinc.com>, 
	Sia Jee Heng <jeeheng.sia@...rfivetech.com>, Björn Töpel <bjorn@...osinc.com>, 
	Song Shuai <suagrfillet@...il.com>, Clément Léger <cleger@...osinc.com>, 
	Al Viro <viro@...iv.linux.org.uk>, Jisheng Zhang <jszhang@...nel.org>, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS

Hi Björn,

On Fri, Mar 8, 2024 at 11:16 AM Björn Töpel <bjorn@...nelorg> wrote:
> >
> > If I remember from Steven's talk, x86 uses dynamically allocated trampolines
> > for per callsite tracers, would CALL_OPS provide better performance than that?
>
> Probably not, and it was really a tongue-in-cheek comment -- nothing I
> encourage you to do!
>
> Now, I think a better approach for RISC-V would be implementing what x86
> has (arch_ftrace_update_trampoline()), rather than CALL_OPS for RISC-V.
>
> Thoughts?

I am going to spin some patches for implementing
arch_ftrace_update_trampoline() for
RISC-V, then we can compare the two approaches and see which is
better. But I agree
that arch_ftrace_update_trampoline() is a better approach given that
we can jump anywhere
with auipc/jalr.

Thanks,
Puranjay