Re: [PATCH v3] fsnotify: optimize the case of no parent watcher

From: Jan Kara <jack@suse.cz>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Jan Kara <jack@suse.cz>, Jens Axboe <axboe@kernel.dk>,
	Christian Brauner <brauner@kernel.org>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v3] fsnotify: optimize the case of no parent watcher
Date: Tue, 16 Jan 2024 13:04:34 +0100	[thread overview]
Message-ID: <20240116120434.gsdg7lhb4pkcppfk@quack3> (raw)
In-Reply-To: <20240116113247.758848-1-amir73il@gmail.com>

On Tue 16-01-24 13:32:47, Amir Goldstein wrote:
> If parent inode is not watching, check for the event in masks of
> sb/mount/inode masks early to optimize out most of the code in
> __fsnotify_parent() and avoid calling fsnotify().
> 
> Jens has reported that this optimization improves BW and IOPS in an
> io_uring benchmark by more than 10% and reduces perf reported CPU usage.
> 
> before:
> 
> +    4.51%  io_uring  [kernel.vmlinux]  [k] fsnotify
> +    3.67%  io_uring  [kernel.vmlinux]  [k] __fsnotify_parent
> 
> after:
> 
> +    2.37%  io_uring  [kernel.vmlinux]  [k] __fsnotify_parent
> 
> Reported-and-tested-by: Jens Axboe <axboe@kernel.dk>
> Link: https://lore.kernel.org/linux-fsdevel/b45bd8ff-5654-4e67-90a6-aad5e6759e0b@kernel.dk/
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> ---
> 
> Jan,
> 
> Considering that this change looks like a clear win and it actually
> the change that you suggested, I cleaned it up a bit and posting for
> your consideration.

Agreed, I like this. What did you generate this patch against? It does not
apply on top of current Linus' tree (maybe it needs the change sitting in
VFS tree - which is fine I can wait until that gets merged)?

> I've kept the wrappers fsnotify_path() and fsnotify_sb_has_watchers()
> although they are not directly related to the optimization, because they
> makes the code a bit nicer IMO.

Yeah, these cleanups are fine. I would have prefered them as a separate
patch (some people might want the performance improvement to be backported
and this makes it unnecessarily more complex) but don't resend just because
of that.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR