linux-kernel - Re: [PATCH] fs/address_space: move i_mmap_rwsem to mitigate a false sharing with i

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Date: Fri, 2 Feb 2024 19:32:36 +0000
From: Matthew Wilcox <willy@...radead.org>
To: JonasZhou-oc <JonasZhou-oc@...oxin.com>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	CobeChen@...oxin.com, LouisQi@...oxin.com, JonasZhou@...oxin.com
Subject: Re: [PATCH] fs/address_space: move i_mmap_rwsem to mitigate a false
 sharing with i_mmap.

On Fri, Feb 02, 2024 at 03:03:51PM +0000, Matthew Wilcox wrote:
> On Fri, Feb 02, 2024 at 05:34:07PM +0800, JonasZhou-oc wrote:
> > 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.
> 
> I'm confused.  i_mmap_rwsem protects i_mmap.  Usually you want the lock
> and the thing it's protecting in the same cacheline.  Why is that not
> the case here?

We actually had this seven months ago:

https://lore.kernel.org/all/20230628105624.150352-1-lipeng.zhu@intel.com/

Unfortunately, no argumentation was forthcoming about *why* this was
the right approach.  All we got was a different patch and an assertion
that it still improved performance.

We need to understand what's going on!  Please don't do the same thing
as the other submitter and just assert that it does.