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

Alexei Starovoitov
2016-03-29T00:25:58Z

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@huawei.com
> [2] http://lkml.kernel.org/g/56F89DCD.1040202@huawei.com
>
> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> Cc: He Kuang <hekuang@huawei.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Zefan Li <lizefan@huawei.com>
> Cc: pi3orama@163.com
> ---
> kernel/events/ring_buffer.c | 42 ++++++++++++++++++++++++++++++++++++------
> 1 file changed, 36 insertions(+), 6 deletions(-)

Acked-by: Alexei Starovoitov <ast@kernel.org>