Re: [PATCH 3/4] perf core: Prepare writing into ring buffer from end

[Alexei Starovoitov]

On Mon, Mar 28, 2016 at 06:41:31AM +0000, Wang Nan wrote:
> Convert perf_output_begin to __perf_output_begin and make the later
> function able to write records from the end of the ring buffer.
> Following commits will utilize the 'backward' flag.
> 
> This is the core patch to support writing ring buffer backward, which
> would be introduced by following patch to support reading from
> overwritable ring buffer.
> 
> In theory, this patch should not introduce any extra performance
> overhead since we use always_inline.
> 
> When CONFIG_OPTIMIZE_INLINING is disabled, the output object is nearly
> identical to original one. See [1].
> 
> When CONFIG_OPTIMIZE_INLINING is enabled, the resuling object file becomes
> smaller:
> 
>  $ size kernel/events/ring_buffer.o*
>    text       data        bss        dec        hex    filename
>    4545          4          8       4557       11cd kernel/events/ring_buffer.o.new
>    4641          4          8       4653       122d kernel/events/ring_buffer.o.old
> 
> Performance result:
> 
> Calling 3000000 times of 'close(-1)', use gettimeofday() to check
> duration.  Use 'perf record -o /dev/null -e raw_syscalls:*' to capture
> system calls. In ns.
> 
> Testing environment:
> 
>  CPU    : Intel(R) Core(TM) i7-4790 CPU @ 3.60GHz
>  Kernel : v4.5.0
> 
>                      MEAN         STDVAR
>   BASE            800214.950    2853.083
>   PRE            2253846.700    9997.014
>   POST           2257495.540    8516.293
> 
> Where 'BASE' is pure performance without capturing. 'PRE' is test
> result of pure 'v4.5.0' kernel. 'POST' is test result after this
> patch. See [4] for detail experimental setup.
> 
> Considering the stdvar, this patch doesn't hurt performance.
> 
> For the detail of testing method, please refer to [2].
> 
> [1] http://lkml.kernel.org/g/56F52E83.70409@xxxxxxxxxx
> [2] http://lkml.kernel.org/g/56F89DCD.1040202@xxxxxxxxxx
> 
> Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
> Cc: He Kuang <hekuang@xxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Brendan Gregg <brendan.d.gregg@xxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Zefan Li <lizefan@xxxxxxxxxx>
> Cc: pi3orama@xxxxxxx
> ---
>  kernel/events/ring_buffer.c | 42 ++++++++++++++++++++++++++++++++++++------
>  1 file changed, 36 insertions(+), 6 deletions(-)

Acked-by: Alexei Starovoitov <ast@xxxxxxxxxx>