On Wed, Nov 05, 2014 at 10:58:28AM +0100, Stephane Eranian wrote: > On Wed, Nov 5, 2014 at 10:21 AM, Peter Zijlstra <pet...@infradead.org> wrote: > > On Tue, Nov 04, 2014 at 09:56:09PM -0500, Kan Liang wrote: > >> From: Yan, Zheng <zheng.z....@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.
But this patch changes the FP callchain interface. I see no need of that. We already have multiple independent callchain options (FP and Dwarf) adding a third option should also be independent (LBR). Allowing all 3 at the same time allows for identifying those corner cases. That is I simply don't see a good reason intertwine these things at the interface level. All it does is reduce options. Would it not be 'nice' to allow both FP and LBR at the same time? -- 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/
