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SubjectRe: [PATCH 00/14] perf, x86: Haswell LBR call stack support
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@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
>>
>> 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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