linux-kernel - Re: [RFC PATCH 0/3] tracing: Introduce relative stacktrace

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Message-ID: <20250128123012.4f425227@gandalf.local.home>
Date: Tue, 28 Jan 2025 12:30:12 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>, Luis Chamberlain
 <mcgrof@...nel.org>, Petr Pavlu <petr.pavlu@...e.com>, Sami Tolvanen
 <samitolvanen@...gle.com>, Daniel Gomez <da.gomez@...sung.com>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 linux-modules@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] tracing: Introduce relative stacktrace

On Tue, 28 Jan 2025 11:46:25 -0500
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> I'm trying to wrap my head around what you are trying to achieve here.
> 
> So AFAIU you are aiming to store the relative offset from kernel _text
> and module base text address into the traced events rather than the
> actual address.
> 
> Based on Masami's cover letter, this appears to be  done to make sure
> users can get to this base+offset information even if they cannot read
> kallsyms.
> 
> Why make the tracing fast path more complex for a simple matter of
> accessing this base address information ?
> 
> All you need to have to convert from kernel address to base + offset is:
> 
> - The kernel _text base address,
> - Each loaded module text base address,
> - Unloaded modules events to prune this information.
> 
> What is wrong with simply exporting this base address information in the
> trace buffers rather than rely on kallsyms, and deal with the conversion
> to module name / base+offset at post-processing ?

Hmm, we could probably get away with that too. I think we were focused on
kallsyms, where we wanted a way to not have to distinguish between current
boot info and previous boot info. But when we started pulling in the module
info, it may be possible to do a post processing.

I have said in the past that I wanted module information in the persistent
memory. By doing that this may not be needed. I'll look into it.

-- Steve