Re: [PATCH v2] buffer: Avoid setting buffer bits that are already set

[kemi <kemi.wang@intel.com>]

On 2017年10月24日 09:16, Kemi Wang wrote:
> It's expensive to set buffer flags that are already set, because that
> causes a costly cache line transition.
> 
> A common case is setting the "verified" flag during ext4 writes.
> This patch checks for the flag being set first.
> 
> With the AIM7/creat-clo benchmark testing on a 48G ramdisk based-on ext4
> file system, we see 3.3%(15431->15936) improvement of aim7.jobs-per-min on
> a 2-sockets broadwell platform.
> 
> What the benchmark does is: it forks 3000 processes, and each  process do
> the following:
> a) open a new file
> b) close the file
> c) delete the file
> until loop=100*1000 times.
> 
> The original patch is contributed by Andi Kleen.
> 
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Kemi Wang <kemi.wang@intel.com>
> Tested-by: Kemi Wang <kemi.wang@intel.com>
> Reviewed-by: Jens Axboe <axboe@kernel.dk>
> ---

Seems that this patch is still not merged. Anything wrong with that? thanks

>  include/linux/buffer_head.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index c8dae55..211d8f5 100644
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -80,11 +80,14 @@ struct buffer_head {
>  /*
>   * macro tricks to expand the set_buffer_foo(), clear_buffer_foo()
>   * and buffer_foo() functions.
> + * To avoid reset buffer flags that are already set, because that causes
> + * a costly cache line transition, check the flag first.
>   */
>  #define BUFFER_FNS(bit, name)						\
>  static __always_inline void set_buffer_##name(struct buffer_head *bh)	\
>  {									\
> -	set_bit(BH_##bit, &(bh)->b_state);				\
> +	if (!test_bit(BH_##bit, &(bh)->b_state))			\
> +		set_bit(BH_##bit, &(bh)->b_state);              \
>  }									\
>  static __always_inline void clear_buffer_##name(struct buffer_head *bh)	\
>  {									\
>