Messages in this thread | | | Date | Thu, 4 Dec 2014 15:23:30 +0100 | From | Jiri Olsa <> | Subject | Re: [PATCH V5 0/3] perf tool: Haswell LBR call stack support (user) |
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On Tue, Dec 02, 2014 at 10:06:51AM -0500, kan.liang@intel.com wrote: > From: Kan Liang <kan.liang@intel.com> > > This is the user space patch for Haswell LBR call stack support. > For many profiling tasks we need the callgraph. For example we often > need to see the caller of a lock or the caller of a memcpy or other > library function to actually tune the program. Frame pointer unwinding > is efficient and works well. But frame pointers are off by default on > 64bit code (and on modern 32bit gccs), so there are many binaries around > that do not use frame pointers. Profiling unchanged production code is > very useful in practice. On some CPUs frame pointer also has a high > cost. Dwarf2 unwinding also does not always work and is extremely slow > (upto 20% overhead). > > 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 provides an alternative to get > callgraph. It has some limitations too, but should work in most cases > and is significantly faster than dwarf. Frame pointer unwinding is still > the best default, but LBR call stack is a good alternative when nothing > else works. > > Please find the kernel part patch at https://lkml.org/lkml/2014/11/6/432 > > Changes since v1 > - Update help document > - Force exclude_user to 0 with warning in LBR call stack > - Dump both lbr and fp info when report -D > - Reconstruct thread__resolve_callchain_sample and split it into two patches > - Use has_branch_callstack function to check LBR call stack available > > Changes since v2 > - Rebase to 025ce5d33373 > > Changes since v3 > - Rebase to cc502c23aadf > - Separated function for lbr call stack sample resolve and print > - Some minor changes according to comments > > Changes since V4 > - Rebase to 09a6a1b > - Falling back to framepointers if LBR not available, and warning user
looks ok to me..
I'll test it once I get hands on Haswel server again, I guess we wait for the kernel change to go in first anyway, right?
thanks, jirka
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