linux-kernel - Re: [PATCH v2 1/2] tracing: fprobe: optimization for entry only case

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Message-Id: <20251014235159.fdfc2444582ea15de822c0b4@kernel.org>
Date: Tue, 14 Oct 2025 23:51:59 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
 jiang.biao@...ux.dev, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v2 1/2] tracing: fprobe: optimization for entry only
 case

Hi Menglong,

I remember why I haven't implement this.

On Fri, 10 Oct 2025 11:38:46 +0800
Menglong Dong <menglong8.dong@...il.com> wrote:

> +
> +static struct ftrace_ops fprobe_ftrace_ops = {
> +	.func	= fprobe_ftrace_entry,
> +	.flags	= FTRACE_OPS_FL_SAVE_REGS,

Actually, this flag is the problem. This can fail fprobe on architecture
which does not support CONFIG_DYNAMIC_FTRACE_WITH_REGS (e.g. arm64, riscv)

 * SAVE_REGS - The ftrace_ops wants regs saved at each function called
 *            and passed to the callback. If this flag is set, but the
 *            architecture does not support passing regs
 *            (CONFIG_DYNAMIC_FTRACE_WITH_REGS is not defined), then the
 *            ftrace_ops will fail to register, unless the next flag
 *            is set.

fgraph has a special entry code for saving ftrace_regs.
So at least we need to fail back to fgraph if arch does not
support CONFIG_DYNAMIC_FTRACE_WITH_REGS.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>