Re: [PATCH v3 00/14] perf, x86: Haswell LBR call stack support

[Stephane Eranian <eranian@google.com>]

Could you add the Reviewed-by: on the patches I already
reviewed? So I focus on the changes you made and continue
testing on my HSW system.

Thanks.

On Tue, Feb 18, 2014 at 7:07 AM, Yan, Zheng <zheng.z.yan@intel.com> 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
>
> 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
>
>