On Wed, Jun 26, 2013 at 1:54 PM, Peter Zijlstra <pet...@infradead.org> wrote: > On Tue, Jun 25, 2013 at 04:47:12PM +0800, Yan, Zheng wrote: >> From: "Yan, Zheng" <zheng.z....@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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
