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

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Date: Wed, 6 Mar 2024 14:16:22 +0800
From: JonasZhou <JonasZhou-oc@...oxin.com>
To: <david@...morbit.com>
CC: <CobeChen@...oxin.com>, <JonasZhou@...oxin.com>, <LouisQi@...oxin.com>,
	<brauner@...nel.org>, <jack@...e.cz>, <jonaszhou-oc@...oxin.com>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<viro@...iv.linux.org.uk>, <willy@...radead.org>
Subject: Re: [PATCH] fs/address_space: move i_mmap_rwsem to mitigate a false sharing with i_mmap.

> On Mon, Feb 05, 2024 at 02:22:29PM +0800, JonasZhou wrote:
> 
> As I expected, your test is exercising the contention case rather
> than the single, uncontended case. As such, your patch is simply
> optimising the structure layout for the contended case at the
> expense of an extra cacheline miss in the uncontended case.
> 
> I'm not an mm expert, so I don't know which case we should optimise
> for.
> 
> However, the existing code is not obviously wrong, it's just that
> your micro-benchmark exercises the pathological worst case for the
> optimisation choices made for this structure. Whether the contention
> case is worth optimising is the first decision that needs to be
> made, then people can decide if hacking minor optimisations into the
> code is better than reworking the locking and/or algorithm to avoid
> the contention altogether is a better direction...

According to https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/log/?qt=grep&q=unixbench, 
many people use Unixbench and submit optimization patches to Linux based 
on its scores. So this is not my micro-benchmark exercises.

When multiple processes open the same file, such as multiple processes 
opening libc.so, there will be contention.

> -Dave.
> -- 
> Dave Chinner
> david@...morbit.com