Messages in this thread | | | Date | Wed, 26 Jun 2013 18:48:20 +0200 | Subject | Re: [PATCH 0/7] perf, x86: Haswell LBR call stack support | From | Stephane Eranian <> |
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On Wed, Jun 26, 2013 at 1:54 PM, Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, Jun 25, 2013 at 04:47:12PM +0800, Yan, Zheng wrote: >> From: "Yan, Zheng" <zheng.z.yan@intel.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. 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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