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

[Matthew Wilcox]

On Mon, Feb 05, 2024 at 02:22:29PM +0800, JonasZhou wrote:
> When running UnixBench/execl, each execl process repeatedly performs 
> i_mmap_lock_write -> vma_interval_tree_remove/insert -> 
> i_mmap_unlock_write. As indicated below, when i_mmap and i_mmap_rwsem 
> are in the same CACHE Line, there will be more HITM.

(I wasn't familiar with the term HITM.  For anyone else who's
unfamiliar, this appears to mean a HIT in another core's cache, which
has the cachline in the Modified state)

> Func0: i_mmap_lock_write
> Func1: vma_interval_tree_remove/insert
> Func2: i_mmap_unlock_write
> In the same CACHE Line
> Process A | Process B | Process C | Process D | CACHE Line state 
> ----------+-----------+-----------+-----------+-----------------
> Func0     |           |           |           | I->M
>           | Func0     |           |           | HITM M->S
> Func1     |           |           |           | may change to M
>           |           | Func0     |           | HITM M->S
> Func2     |           |           |           | S->M
>           |           |           | Func0     | HITM M->S
> 
> In different CACHE Lines
> Process A | Process B | Process C | Process D | CACHE Line state 
> ----------+-----------+-----------+-----------+-----------------
> Func0     |           |           |           | I->M
>           | Func0     |           |           | HITM M->S
> Func1     |           |           |           | 
>           |           | Func0     |           | S->S
> Func2     |           |           |           | S->M
>           |           |           | Func0     | HITM M->S
> 
> The same issue will occur in Unixbench/shell because the shell 
> launches a lot of shell commands, loads executable files and dynamic 
> libraries into memory, execute, and exit.

OK, I see.

> Yes, his commit has been merged into the Linux kernel, but there 
> is an issue. After moving i_mmap_rwsem below flags, there is a 
> 32-byte gap between i_mmap_rwsem and i_mmap. However, the struct 
> address_space is aligned to sizeof(long), which is 8 on the x86-64 
> architecture. As a result, i_mmap_rwsem and i_mmap may be placed on 
> the same CACHE Line, causing a false sharing problem. This issue has 
> been observed using the perf c2c tool.

Got it.  OK, let's put this patch in.  It's a stopgap measure, clearly.
I'll reply to Dave's email with a longer term solution.