Re: [PATCH] fs/address_space: move i_mmap_rwsem to mitigate a false sharing with i_mmap.

[Al Viro]

On Fri, Feb 02, 2024 at 04:33:04PM +0800, JonasZhou-oc wrote:
> From: JonasZhou <JonasZhou@xxxxxxxxxxx>
> 
> In the struct address_space, there is a 32-byte gap between i_mmap
> and i_mmap_rwsem. Due to the alignment of struct address_space
> variables to 8 bytes, in certain situations, i_mmap and i_mmap_rwsem
> may end up in the same CACHE line.
> 
> While running Unixbench/execl, we observe high false sharing issues
> when accessing i_mmap against i_mmap_rwsem. We move i_mmap_rwsem
> after i_private_list, ensuring a 64-byte gap between i_mmap and
> i_mmap_rwsem.
> 
> For Intel Silver machines (2 sockets) using kernel v6.8 rc-2, the score
> of Unixbench/execl improves by ~3.94%, and the score of Unixbench/shell
> improves by ~3.26%.

Looks sane.