linux-kernel - Re: [tracing] 06e0a548ba: WARNING:at_kernel/trace/ring_buffer.c:#ring_buffer_iter

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Message-ID: <20200513161557.GA73453@tuxmaker.boeblingen.de.ibm.com>
Date:   Wed, 13 May 2020 18:15:57 +0200
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org
Subject: Re: [tracing] 06e0a548ba:
 WARNING:at_kernel/trace/ring_buffer.c:#ring_buffer_iter_peek

Hi Steve,

On Wed, May 13, 2020 at 09:29:22AM -0400, Steven Rostedt wrote:
> On Wed, 13 May 2020 11:19:06 +0200
> Sven Schnelle <svens@...ux.ibm.com> wrote:
> 
> > Did you had a chance to look into this? I can easily reproduce this both on x86
> > and s390 by doing:
> > 
> > cd /sys/kernel/tracing
> > cat /dev/zero >/dev/null & # generate some load
> > echo function >current_tracer
> > # wait a few seconds to fill the buffer
> > cat trace
> > 
> > Usually it will print the warn after a few seconds.
> > 
> > I haven't digged through all the ring buffer code yet, so i thought i might ask
> > whether you have an idea what's going on.
> 
> Can you send me the config for where you can reproduce it on x86?
> 
> The iterator now doesn't stop the ring buffer when it iterates, and is
> doing so over a live buffer (but should be able to handle it). It's
> triggering something I thought wasn't suppose to happen (which must be
> happening).
> 
> Perhaps with your config I'd be able to reproduce it.

Thanks for looking into this. I've attached my /proc/config.gz to this Mail.
The x86 system is my Laptop which is a Thinkpad X280 with 4 HT CPUs (so 8 cpus
in total). I've tried disabling preemption, but this didn't help.

It's always this check that causes the loop:

if (iter->head >= rb_page_size(iter->head_page)) {
	rb_inc_iter(iter);
	goto again;
}

On the first loop iter->head is some value > 0 and rb_page_size returns
0, afterwards it gets twice to this check with both values 0. The third
time the warning is triggered. Maybe that information helps.

Thanks,
Sven

Download attachment "config.gz" of type "application/gzip" (29532 bytes)