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

[Stephane Eranian]

On Wed, Jun 26, 2013 at 1:54 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, Jun 25, 2013 at 04:47:12PM +0800, Yan, Zheng wrote:
>> From: "Yan, Zheng" <zheng.z.yan@xxxxxxxxx>
>>
>> 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. When perf tool requests PERF_SAMPLE_CALLCHAIN +
>> PERF_SAMPLE_BRANCH_USER, this feature is dynamically enabled by default.
>> This feature can be disabled/enabled through an attribute file in the cpu
>> pmu sysfs directory.
>>
>> 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
>>
>
> You fail to mention what happens when the callstack is deeper than the
> LBR is big -- a rather common issue I'd think.
>
LBR is statistical callstack. By nature, it cannot capture the entire chain.

> From what I gather if you push when full, the TOS rotates and eats the
> tail allowing you to add another entry to the head.
>
> If you pop when empty; nothing happens.
>
Not sure they know "empty" from "non empty", they just move the LBR_TOS
by one entry on returns.

> So on pretty much every program you'd be lucky to get the top of the
> callstack but can end up with nearly nothing.
>
You will get the calls closest to the interrupt.

> Given that, and the other limitations I don't think its a fair
> replacement for user callchains.

Well, the one advantage I see is that it works on stripped/optimized
binaries without fp or dwarf info. Compared to dwarf and the stack
snapshot, it does incur less overhead most likely. But yes, it comes
with limitations.
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