linux-kernel - RE: [patch 3/5] x86, ftrace, hw-branch-tracer: dump on oops

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Message-ID: <928CFBE8E7CB0040959E56B4EA41A77E0CDD7B78@irsmsx504.ger.corp.intel.com>
Date:	Fri, 16 Jan 2009 15:03:48 +0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Steven Rostedt <srostedt@...hat.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"markus.t.metzger@...il.com" <markus.t.metzger@...il.com>
Subject: RE: [patch 3/5] x86, ftrace, hw-branch-tracer: dump on oops

>-----Original Message-----
>From: Steven Rostedt [mailto:srostedt@...hat.com]
>Sent: Friday, January 16, 2009 3:55 PM
>To: Metzger, Markus T
>Cc: linux-kernel@...r.kernel.org; mingo@...e.hu; tglx@...utronix.de; hpa@...or.com;
>markus.t.metzger@...il.com
>Subject: Re: [patch 3/5] x86, ftrace, hw-branch-tracer: dump on oops
>
>BTW, you have two 1/5 patches but no 2/5 patch.
>
>On Fri, 2009-01-16 at 11:58 +0100, Markus Metzger wrote:
>> Dump the branch trace on an oops (based on ftrace_dump_on_oops).
>
>Does the ftrace_dump_on_oops not work?  We may want to change
>ftrace_dump to see if a tracer has been registered, and use its format
>for output.

It does work and I am using it.
It only works one time, though. Ftrace kills itself before dumping its buffer.

The trace is collected in a separate buffer. trace_hw_branch_oops() reads that buffer and adds the trace to the ftrace buffer, so ftrace can then dump it.


>>       bts_trace_stop(tr);
>>       unregister_hotcpu_notifier(&bts_hotcpu_notifier);
>> +
>> +     hw_branch_trace = NULL;
>
>unless you test hw_branch_trace for content, I usually never NULL the
>local tracer variable. This prevents races. The worst that can happen
>without nulling it, is that your tracer will write into another tracer's
>buffer. No real harm done. But if you have it as null and a race
>happens, you can crash the kernel.

Sounds reasonable. I'll change it.

regards,
markus.

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