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SubjectRe: [PATCH v5 00/16] perf, x86: Haswell LBR call stack support
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On Fri, Sep 5, 2014 at 4:25 PM, Liang, Kan <kan.liang@intel.com> wrote:
> Hi Peter and all,
>
> Did you get a chance to review these patches?
> Zheng is away. Should I re-send the patches?
>
Please resubmit rebased to tip.git and I'll take another look.

> 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
>> --
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