linux-kernel - Re: [PATCH v1] dynamic_debug: add support for logs destination

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Date:   Mon, 16 Oct 2023 11:03:12 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Pekka Paalanen <ppaalanen@...il.com>, jim.cromie@...il.com,
        Łukasz Bartosik <lb@...ihalf.com>,
        linux-kernel@...r.kernel.org,
        "wayland-devel@...ts.freedesktop.org" 
        <wayland-devel@...ts.freedesktop.org>,
        Sean Paul <seanpaul@...omium.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v1] dynamic_debug: add support for logs destination

On Thu, 12 Oct 2023 11:53:52 +0200
Daniel Vetter <daniel@...ll.ch> wrote:

> > You said that turning the kernel ring buffer contents into strings is a
> > very heavy operation, so it is not possible to push this scope
> > separation to userspace, right?  
> 
> I think it's the kernel that does the formatting, but honestly not sure
> how this works with perf traces. Might be that it's actually userspace
> doing the formatting later on so that it doesn't incur the overhead while
> recording.

perf and trace-cmd do the formatting in user space via libtraceevent:

  git://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git

It reads the format files of the events:

  /sys/kernel/tracing/events/*/*/format

and uses that to read the raw data saved from the kernel into human
readable output.

Note, this means that addresses coming from kernel trace events are not
hashed!

-- Steve