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

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Date:	Wed, 24 Oct 2012 14:36:19 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	"Yan, Zheng" <zheng.z.yan@...el.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 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...


> See it as a "prefer the needs of the 95+% over the needs of the few" default
>
> -Andi
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