linux-kernel - Re: [PATCH] s390:ftrace: add save_stack_trace

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Message-ID: <20160129102241.1b3c9c04@gandalf.local.home>
Date:	Fri, 29 Jan 2016 10:22:41 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Pratyush Anand <panand@...hat.com>, schwidefsky@...ibm.com,
	linux-s390@...r.kernel.org, Chunyu Hu <chuhu@...hat.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] s390:ftrace: add save_stack_trace_regs()

On Fri, 29 Jan 2016 15:45:22 +0100
Heiko Carstens <heiko.carstens@...ibm.com> wrote:

> On Fri, Jan 29, 2016 at 08:59:36AM -0500, Steven Rostedt wrote:
> > On Fri, 29 Jan 2016 13:57:49 +0100
> > Heiko Carstens <heiko.carstens@...ibm.com> wrote:
> >   
> > > Surpringly it wasn't one of my own patches which broke the stack tracer on
> > > s390, but one from Steven:
> > > 
> > > 72ac426a5bb0 ("tracing: Clean up stack tracing and fix fentry updates")
> > > 
> > > Now I only need to figure out why :)  
> > 
> > Try this one?
> > 
> >   7717c6be699975f6733d278b13b7c4295d73caf6
> >   tracing: Fix stacktrace skip depth in trace_buffer_unlock_commit_regs()
> > 
> > Or at the very least, merge with Linus's latest and see if something
> > else doesn't fix it.  
> 
> No, that doesn't fix it (current Linus' master):
> 
> # uname -a
> Linux p2345007 4.5.0-rc1-00032-g26cd83670f2f #26 SMP Fri Jan 29 15:39:47 CET 2016 s390x s390x s390x GNU/Linux
> 
> # cat stack_max_size 
> 4496
> 
> # cat stack_trace
>         Depth    Size   Location    (0 entries)
>         -----    ----   --------

Ah this stack trace. This is different than what Pratyush Anand is
fixing. This is the stack tracer which uses function tracing. The other
fix is to deal with stack traces from events, specifically kprobes. As
he stated, you test with:

# echo stacktrace > /sys/kernel/debug/tracing/trace_options
# echo "p kfree" >> /sys/kernel/debug/tracing/kprobe_events
# echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable

But anyway, I'm curious, does this fix the issue for you?

-- Steve

diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index dda9e6742950..db1c26c385c7 100644
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -125,6 +125,9 @@ check_stack(unsigned long ip, unsigned long *stack)
 			break;
 	}
 
+	if (i == stack_trace_max.nr_entries)
+		i = 0;
+
 	/*
 	 * Now find where in the stack these are.
 	 */