Re: [PATCH 00/14] perf, x86: Haswell LBR call stack support

[Yan, Zheng]

On 01/21/2014 09:17 PM, Stephane Eranian wrote:
> Hi,
> 
> Is there a git tree from which I could could pull those 14 patches from?

https://github.com/ukernel/linux.git perf-lbr-callstack

Regards
Yan, Zheng

> 
> On Fri, Jan 3, 2014 at 6:47 AM, Yan, Zheng <zheng.z.yan@xxxxxxxxx> wrote:
>> 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.
>>
>> This patch series adds LBR call stack support. User can enabled/disable
>> this through an sysfs attribute file in the CPU PMU directory:
>>  echo 1 > /sys/bus/event_source/devices/cpu/lbr_callstack
>>
>> 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
>>
>> Change since previous version
>>  - 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
>>

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