Hi Peter and all, Did you get a chance to review these patches? Zheng is away. Should I re-send the patches?
Thanks, Kan > > 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. > > When profiling bc(1) on Fedora 19: > echo 'scale=2000; 4*a(1)' > cmd; perf record -g fp bc -l < cmd > > If this feature is enabled, 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 this feature is disabled, perf report output looks like: > 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 > > 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 > - split change into more patches > - introduce context switch callback and use it to flush LBR > - use the context switch callback to save/restore LBR > - dynamic allocate memory area for storing LBR stack, always switch the > memory area during context switch > - disable this feature by default > - more description in change logs > > Changes since v2 > - don't use xchg to switch PMU specific data > - remove nr_branch_stack from struct perf_event_context > - simplify the save/restore LBR stack logical > - remove unnecessary 'has_branch_stack -> needs_branch_stack' > conversion > - more description in change logs > > Changes since v3 > - remove sysfs attribute file that disable this feature > > Changes since v4 > - re-organize code that save/resotre LBR stack > - allocate pmu specific data when it's needed > - update code comments > > These patches are also available at: > > These patches are also available at: > https://github.com/ukernel/linux.git perf-lbr-callstack > -- > 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/ -- 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/
