linux-kernel - RE: [patch] x86, bts: use atomic memory allocation

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Date:	Wed, 25 Mar 2009 09:07:41 +0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"markus.t.metzger@...il.com" <markus.t.metzger@...il.com>,
	"roland@...hat.com" <roland@...hat.com>,
	"eranian@...glemail.com" <eranian@...glemail.com>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"Villacis, Juan" <juan.villacis@...el.com>,
	"ak@...ux.jf.intel.com" <ak@...ux.jf.intel.com>
Subject: RE: [patch] x86, bts: use atomic memory allocation

>-----Original Message-----
>From: Ingo Molnar [mailto:mingo@...e.hu]
>Sent: Friday, March 20, 2009 7:56 PM
>To: Metzger, Markus T

>> >> Ds_request_bts() needs to allocate memory. It uses GFP_KERNEL.
>> >>
>> >> Hw-branch-tracer calls ds_request_bts() within on_each_cpu().
>> >>
>> >> Use atomic memory allocation to allow it to be used in that context.
>> >
>> >the hw-branch-tracer still crashes during bootup. Have you tried the
>> >config i sent to you, and have you tried to reproduce it? I've
>> >attached another config that crashes.
>>
>> The first config boots OK.
>
>> The second config boots OK with the additional changes to keep the
>> GFP_KERNEL and move the ds_request_bts() calls out of the
>> on_each_cpu() in the hw-branch-tracer.
>>
>> I don't know yet what exactly causes the crash and if there is a
>> simpler fix.
>
>hm, this was an AMD box - so perhaps they dont have a compatible DS
>implementation?

If no trace hardware is detected, the hw-branch tracer returns -EOPNOTSUPP.
I doubt that there is an incompatible DS implementation.

I found a bug that leads to a kernel crash.

If the tracing task exits, it disables bts tracing for all its children and then
frees the trace buffer. If the children are currently executing, the buffer
may be freed while the hardware is still tracing. This might cause the hardware to
overwrite memory.

I would need to wait until the traced task is rescheduled, trigger a rescheduling,
or actively disable tracing on the traced task's cpu.

Unfortunately, most of the ptrace exit code is executed with irq's disabled and
tasklist_lock write-held.
I am currently trying to find a way to do this at a time when irq's are enabled.

This affects the ptrace bts extension for .29 and tip/tracing/hw-branch-tracing.
The hw-branch-tracer is not affected.

regards,
markus.
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