Messages in this thread | | | From | kan.liang@intel ... | Subject | [PATCH V3 0/3] perf tool: Haswell LBR call stack support (user) | Date | Fri, 14 Nov 2014 08:44:09 -0500 |
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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.
A new call chain recording option "lbr" is introduced into perf tool for LBR call stack. The user can use --call-graph lbr to get the call stack information from hardware.
When profiling bc(1) on Fedora 19: echo 'scale=2000; 4*a(1)' > cmd; perf record --call-graph lbr bc -l < cmd If enabling LBR, perf report output looks like: 50.36% bc bc [.] bc_divide | --- bc_divide execute run_code yyparse main __libc_start_main _start 33.66% bc bc [.] _one_mult | --- _one_mult bc_divide execute run_code yyparse main __libc_start_main _start 7.62% bc bc [.] _bc_do_add | --- _bc_do_add | |--99.89%-- 0x2000186a8 --0.11%-- [...] 6.83% bc bc [.] _bc_do_sub | --- _bc_do_sub | |--99.94%-- bc_add | execute | run_code | yyparse | main | __libc_start_main | _start --0.06%-- [...] 0.46% bc libc-2.17.so [.] __memset_sse2 | --- __memset_sse2 | |--54.13%-- bc_new_num | | | |--51.00%-- bc_divide | | execute | | run_code | | yyparse | | main | | __libc_start_main | | _start | | | |--30.46%-- _bc_do_sub | | bc_add | | execute | | run_code | | yyparse | | main | | __libc_start_main | | _start | | | --18.55%-- _bc_do_add | bc_add | execute | run_code | yyparse | main | __libc_start_main | _start | --45.87%-- bc_divide execute run_code yyparse main __libc_start_main _start If using FP, perf report output looks like: echo 'scale=2000; 4*a(1)' > cmd; perf record --call-graph fp bc -l < cmd 50.49% bc bc [.] bc_divide | --- bc_divide 33.57% bc bc [.] _one_mult | --- _one_mult 7.61% bc bc [.] _bc_do_add | --- _bc_do_add 0x2000186a8 6.88% bc bc [.] _bc_do_sub | --- _bc_do_sub 0.42% bc libc-2.17.so [.] __memcpy_ssse3_back | --- __memcpy_ssse3_back
If using LBR, perf report -D output looks like: 11739295893248 0x4d0 [0xe0]: PERF_RECORD_SAMPLE(IP, 0x2): 10505/10505: 0x40054d period: 39255 addr: 0 ... LBR call chain: nr:7 ..... 0: fffffffffffffe00 ..... 1: 0000000000400540 ..... 2: 0000000000400587 ..... 3: 00000000004005b3 ..... 4: 00000000004005ef ..... 5: 0000003d1cc21b43 ..... 6: 0000000000400474 ... FP chain: nr:6 ..... 0: fffffffffffffe00 ..... 1: 000000000040054d ..... 2: 000000000040058c ..... 3: 00000000004005b8 ..... 4: 00000000004005f4 ..... 5: 0000003d1cc21b45 ... thread: a.out:10505 ...... dso: /home/lk/a.out
The LBR call stack has following known limitations - Zero length calls are not filtered out by hardware - Exception handing such as setjmp/longjmp will have calls/returns not match - Pushing different return address onto the stack will have calls/returns not match - If callstack is deeper than the LBR, only the last entries are captured
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
Kan Liang (3): perf tools: enable LBR call stack support perf tool: Move cpumode resolve code to add_callchain_ip perf tools: Construct LBR call chain
tools/perf/Documentation/perf-record.txt | 8 +- tools/perf/builtin-record.c | 6 +- tools/perf/builtin-report.c | 2 + tools/perf/util/callchain.c | 10 ++- tools/perf/util/callchain.h | 1 + tools/perf/util/evsel.c | 21 ++++- tools/perf/util/evsel.h | 4 + tools/perf/util/machine.c | 134 +++++++++++++++++++++---------- tools/perf/util/session.c | 60 ++++++++++++-- 9 files changed, 192 insertions(+), 54 deletions(-)
-- 1.8.3.2
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