linux-kernel - Re: [PATCH V2 0/7] perf, x86: Haswell LBR call stack support

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Message-ID: <CABPqkBS320VrPL9CzDVMzQCkHb6sxC+d1d7f_J3Mp0UXmF=3XA@mail.gmail.com>
Date:	Wed, 24 Oct 2012 10:49:08 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	"Yan, Zheng" <zheng.z.yan@...el.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>
Subject: Re: [PATCH V2 0/7] perf, x86: Haswell LBR call stack support

On Wed, Oct 24, 2012 at 7:59 AM, Yan, Zheng <zheng.z.yan@...el.com> wrote:
> From: "Yan, Zheng" <zheng.z.yan@...el.com>
>
> Haswell has a new feature that utilizes the existing Last Branch Record
> facility to record call chains. When the feature is enabled, function
> call will be collected as normal, but as return instructions are executed
> the last captured branch record is popped from the on-chip LBR registers.
> The LBR call stack facility can help perf to get call chains of progam
> without frame pointer.
>
> The LBR call stack has following known limitations
>  1. Zero length calls are not filtered out by hardware
>  2. Exception handing such as setjmp/longjmp will have calls/returns not
>     match
>  3. Pushing different return address onto the stack will have calls/returns
>     not match
>
> The patch series depends on Andi's "perf PMU support for Haswel" patches
>
The part that is missing here is that you don't explain that you are now
fitting LBR cstack under PERF_SAMPLE_CALLCHAIN. It is dynamically
enabled based on users callstack request mode.

> ---
> Changes since v1
>  - not expose PERF_SAMPLE_BRANCH_CALL_STACK to user space
>  - save/restore LBR stack on context switch for all sampling branch modes
>  - reduce lbr_sel_map size
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