linux-kernel - Re: [PATCH V2 6/7] perf, x86: Use LBR call stack to get user callchain

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Message-ID: <5087E323.3020501@intel.com>
Date:	Wed, 24 Oct 2012 20:46:27 +0800
From:	"Yan, Zheng" <zheng.z.yan@...el.com>
To:	Stephane Eranian <eranian@...gle.com>
CC:	Andi Kleen <ak@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH V2 6/7] perf, x86: Use LBR call stack to get user callchain

On 10/24/2012 08:36 PM, Stephane Eranian wrote:
> On Wed, Oct 24, 2012 at 2:31 PM, Andi Kleen <ak@...ux.intel.com> wrote:
>> On Wed, Oct 24, 2012 at 02:21:43PM +0200, Stephane Eranian wrote:
>>> But still, if I do:
>>> $  perf record -g -e cpu/cycles/k ........
>>>
>>> Looks like your code will activate LBR cstack for user stack even
>>> though I don't use it.
>>> I know it won't generate any user samples (theoretically) but you are
>>> still commandeering
>>> the LBR resource which other events may want to use.
>>
>> You can disable it in sysfs. LBR is a power user feature and they will
>> know how to do this.
>>
> But isn't the whole point of the patch to hide LBR cstack from users so they
> won't know they're using it for user cstack?
> 
> To disable it in sysfs, I need to know it's being used in the first place.
> You're saying, I need to know it is used only for user cstack and if I don't
> need it then I need to disable it explicitly. I also suspect using it
> for nothing
> may have some performance implication because you are saving/restoring
> on ctxsw, for instance.
> 
> I think in x86_pmu_hw_config(), you could simply check the priv level
> restrictions on the event. If exclude_user is set, then don't active LBR cstack
> and that's it. That's what I was trying to get to...
> 

I will add that check

Thanks
Yan, Zheng

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