Messages in this thread | | | Date | Thu, 27 Jun 2013 16:07:14 +0800 | From | "Yan, Zheng" <> | Subject | Re: [PATCH 0/7] perf, x86: Haswell LBR call stack support |
| |
On 06/27/2013 12:48 AM, Stephane Eranian wrote: > 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.
When pop, it decreases LBR_TOS by one and clear the popped LBR_FROM/LBR_TO MSRs. If pop when empty, you will get an empty callchains.
Regards Yan, Zheng
> >> 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. >
| |