Messages in this thread | | | Date | Wed, 5 Nov 2014 10:58:28 +0100 | Subject | Re: [PATCH V7 13/17] perf, x86: enable LBR callstack when recording callchain | From | Stephane Eranian <> |
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On Wed, Nov 5, 2014 at 10:21 AM, Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, Nov 04, 2014 at 09:56:09PM -0500, Kan Liang wrote: >> From: Yan, Zheng <zheng.z.yan@intel.com> >> >> Only enable LBR callstack when user requires fp callgraph. The feature >> is not available when PERF_SAMPLE_BRANCH_STACK or PERF_SAMPLE_STACK_USER >> is required. >> Also, this feature only affects how to get user callchain. The kernel >> callchain is always got by frame pointers. > > Since FP callchains should not change, this doesn't appear to make any > sense either.
If I recall earlier discussion, the FP callchain are not changed. On HSW, when requesting fp at the user level only, then the kernel automatically tries to use the LBR callstack mode. Advantage is that the user app does not require frame-pointer or dwarf debug info to get correct callchains with perf record. The downside is that LBR callstack does not work in certain callchain corner cases.
The reason why using LBR call stack mode is restricted to user level only is because of a bug in the LBR call stack hardware which forces the kernel to drop LBR_FREEZE_PMI. In other words ,the LBR does not stop on counter overflow, thus it will be wiped out by execution of kernel code leading to the PMU interrupt handler.
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