linux-kernel - Re: [patch 1/5] x86, bts: detect size of DS fields

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Date:	Tue, 17 Mar 2009 16:15:36 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Metzger, Markus T" <markus.t.metzger@...el.com>
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 1/5] x86, bts: detect size of DS fields


* Metzger, Markus T <markus.t.metzger@...el.com> wrote:

> >-----Original Message-----
> >From: Metzger, Markus T
> >Sent: Tuesday, March 17, 2009 1:58 PM
> >To: Ingo Molnar
> 
> 
> >>> I think I found the problem:
> >>>   ds_request_bts() needs to allocate a small amount of memory.
> >>>   It uses GFP_KERNEL.
> >>>
> >>> Hw-branch-tracer essentially does on_each_cpu(ds_request_bts()).
> >>>
> >>> Since ds_request_bts() is meant to be called on the cpu that
> >>> should be traced, it should use GFP_ATOMIC to allow calls from
> >>> smp_call_function() and friends.
> >>>
> >>> Would that be acceptable?
> >>
> >>That's not really a good solution - GFP_ATOMIC is not a reliable
> >>form of allocation.
> 
> That problem also is in the .29-rc version.
> It affects the hw-branch-tracer.
> The ptrace interface should not be affected.
> 
> The changes I sketched below will be too intrusive.
> Do you want a small patch that turns GFP_KERNEL into GFP_ATOMIC
> for .29?

Yeah - but it will be for .29.1 - the hw-branch-tracer is off by 
default.

	Ingo
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