linux-kernel - Re: debugctl msr

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Message-ID: <7c86c4470811210047u66a4c188nb99785527f1718ef@mail.gmail.com>
Date:	Fri, 21 Nov 2008 09:47:50 +0100
From:	"stephane eranian" <eranian@...glemail.com>
To:	"Metzger, Markus T" <markus.t.metzger@...el.com>
Cc:	"Markus Metzger" <markus.t.metzger@...glemail.com>,
	"Ingo Molnar" <mingo@...e.hu>, "Andi Kleen" <andi@...stfloor.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: debugctl msr

Markus,

On Fri, Nov 21, 2008 at 9:22 AM, Metzger, Markus T
<markus.t.metzger@...el.com> wrote:
>>-----Original Message-----
>>From: stephane eranian [mailto:eranian@...glemail.com]
>>Sent: Donnerstag, 20. November 2008 22:19
>>To: Markus Metzger
>
>>- in ds_put_context(), you need to mark task->thread.ds_ctx = NULL or
>>this_system_context = NULL when you are
>>  done, otherwise a subsequent session on the same task or CPU will
>>think there is already a context allocated but
>>  the pointer will be stale.
>
> This should happen in:
>    *(context->this) = NULL;
>
Ah, I missed that. I admit this code is hard to follow. Maybe it would easier to
explicitly use the actual fields.
>
>>- in ptrace.c:ptrace_disable(), you systematically invoke ds_release()
>>without checking if TIF_BTS_TRACE_TS
>>  is set. That causes extraneous calls to ds_release() which messes up
>>the reference counting if PEBS is in use.
>
> That TIF only triggers the recording of scheduling timestamps.
>
> That ds_release_bts() call should:
> - get the context
> - fail the ownership validation in ds_validate()
> - put the context
>
> If ds_validate() is disabled, it would put the context twice.
>
Yes, that was my case.


>
> I am currently working on a patch to replace the validation
> mechanism by using a handle returned from ds_request().
>
Yes, that's the solution.

> I also added the interrupt threshold to the parameters of ds_request().
> It will be fix for one tracing session, if that's OK with you.
>
Yes.

I found the reason why in system-wide my kernel was crashing.
It was due to the way you are writing DS_AREA_MSR in
ds_allocate_context():

                if (!task || (task == current))
                        wrmsr(MSR_IA32_DS_AREA, (unsigned long)context->ds, 0);


This does not work in 64-bit, you truncate the DS address. You must use wrmsrl()
and let the macro break it down in two halves.

I am still seeing an issue whereby I have a buffer with > 100 entries
but I get an interrupt
or each sample, which is wrong. I am investigating why this is. There
should only be one
interrupt per buffer full. This use to work with my old ways of handling PEBS.

Thanks.
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