linux-kernel - Re: [PATCHv2][SMB3] Add kernel trace support

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Message-ID: <20180519232257.GL10363@dastard>
Date:   Sun, 20 May 2018 09:22:57 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Steve French <smfrench@...il.com>
Cc:     Ralph Böhme <slow@...ba.org>,
        CIFS <linux-cifs@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        samba-technical <samba-technical@...ts.samba.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCHv2][SMB3] Add kernel trace support

On Fri, May 18, 2018 at 01:43:14PM -0700, Steve French wrote:
> On Fri, May 18, 2018 at 11:46 AM, Ralph Böhme <slow@...ba.org> wrote:
> > On Thu, May 17, 2018 at 09:36:36PM -0500, Steve French via samba-technical wrote:
> >> Patch updated with additional tracepoint locations and some formatting
> >> improvements. There are some obvious additional tracepoints that could
> >> be added, but this should be a reasonable group to start with.
> >>
> >> From edc02d6f9dc24963d510c7ef59067428d3b082d3 Mon Sep 17 00:00:00 2001
> >> From: Steve French <stfrench@...rosoft.com>
> >> Date: Thu, 17 May 2018 21:16:55 -0500
> >> Subject: [PATCH] smb3: Add ftrace tracepoints for improved SMB3 debugging
> >>
> >> Although dmesg logs and wireshark network traces can be
> >> helpful, being able to dynamically enable/disable tracepoints
> >> (in this case via the kernel ftrace mechanism) can also be
> >> helpful in more quickly debugging problems, and more
> >> selectively tracing the events related to the bug report.
> >>
> >> This patch adds 12 ftrace tracepoints to cifs.ko for SMB3 events
> >> in some obvious locations.  Subsequent patches will add more
> >> as needed.
> >>
> >> Example use:
> >>    trace-cmd record -e cifs
> >>    <run test case>
> >>    trace-cmd show
> >
> > pardon my ignorance, but are these tracepoints usable with other tracing
> > frameworks like Systemtap?
> >
> > Last time I checked, Systemtap looked like *the* tool.

Systemtap is great when you have a need for custom tracing. But for
day-to-day kernel development, tracepoints are far more useful
because they are always there and can cover all the common
situations that you need to trace.

And when it comes to debugging a one-off user problem when the user
knows nothing about systemtap?  Nothing beats asking the user
to run a trace on built-in tracepoints, reproduce the problem and
send the trace report back as per the above example.

> > Is there a generic trace
> > point infrastructure that tracing tools can consume, so we're not tied to
> > ftrace?
> 
> At the kernel filesystem/mm summit a few recommended using ftrace
> (trace-cmd).  Don't know what
> the thinking is about this vs. systemtap these days.  There was a nice
> three part series
> describing ftrace/trace-cmd on lwn
> (https://old.lwn.net/Articles/365835/) a while ago.
> 
> In terms of useability "trace-cmd" looked good to me and much more
> powerful than the
> current dmesg based printk style debugging.

And then you learn about trace_printk() for putting custom one-off
debug into the tracepoint stream and wonder why you didn't know
about this years ago :P

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com