linux-kernel - Re: [BUG] msr-trace.h:42 suspicious rcu_dereference

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Message-ID: <20161122190550.GZ26852@two.firstfloor.org>
Date:   Tue, 22 Nov 2016 11:05:50 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Andi Kleen <andi@...stfloor.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Olsa <jolsa@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Jan Stancek <jstancek@...hat.com>
Subject: Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage!

On Tue, Nov 22, 2016 at 09:39:45AM -0500, Steven Rostedt wrote:
> On Mon, 21 Nov 2016 10:37:00 -0800
> Andi Kleen <andi@...stfloor.org> wrote:
> 
> 
> > Here is it again untruncated:
> > 
> > http://halobates.de/tracepoint-trace
> 
> BTW, what tool did you use to generate this?

perf and Intel PT, with the disassembler patches added and udis86
installed:

https://git.kernel.org/cgit/linux/kernel/git/ak/linux-misc.git/log/?h=perf/disassembler-2
http://udis86.sourceforge.net/

You need a CPU that supports PT, like Broadwell, Skylake, Goldmont.

# enable trace point
perf record -e intel_pt// -a sleep X
perf script --itrace=i0ns --ns -F cpu,ip,time,asm,sym

On Skylake can also get more accurate timing by enabling cycle timing
(at the cost of some overhead)

perf record -e intel_pt/cyc=1,cyc_thresh=1/ -a ...

The earlier trace didn't have that enabled.

-Andi