Re: [PATCH 1/3] fs: optimize inotify/fsnotify code for unwatched files

[Jan Kara <jack@suse.cz>]

On Tue 30-06-15 15:04:55, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@linux.intel.com>
> 
> I have a _tiny_ microbenchmark that sits in a loop and writes
> single bytes to a file.  Writing one byte to a tmpfs file is
> around 2x slower than reading one byte from a file, which is a
> _bit_ more than I expecte.  This is a dumb benchmark, but I think
> it's hard to deny that write() is a hot path and we should avoid
> unnecessary overhead there.
> 
> I did a 'perf record' of 30-second samples of read and write.
> The top item in a diffprofile is srcu_read_lock() from
> fsnotify().  There are active inotify fd's from systemd, but
> nothing is actually listening to the file or its part of
> the filesystem.
> 
> I *think* we can avoid taking the srcu_read_lock() for the
> common case where there are no actual marks on the file.
> This means that there will both be nothing to notify for
> *and* implies that there is no need for clearing the ignore
> mask.
> 
> This patch gave a 13.1% speedup in writes/second on my test,
> which is an improvement from the 10.8% that I saw with the
> last version.
> 
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Eric Paris <eparis@redhat.com>
> Cc: John McCutchan <john@johnmccutchan.com>
> Cc: Robert Love <rlove@rlove.org>
> Cc: Andi Kleen <ak@linux.intel.com>
> Cc: linux-kernel@vger.kernel.org

Looks good. You can add:

Reviewed-by: Jan Kara <jack@suse.com>

								Honza

> ---
> 
>  b/fs/notify/fsnotify.c |   10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff -puN fs/notify/fsnotify.c~optimize-fsnotify fs/notify/fsnotify.c
> --- a/fs/notify/fsnotify.c~optimize-fsnotify	2015-06-30 15:03:56.819399401 -0700
> +++ b/fs/notify/fsnotify.c	2015-06-30 15:03:56.822399536 -0700
> @@ -205,6 +205,16 @@ int fsnotify(struct inode *to_tell, __u3
>  		mnt = NULL;
>  
>  	/*
> +	 * Optimization: srcu_read_lock() has a memory barrier which can
> +	 * be expensive.  It protects walking the *_fsnotify_marks lists.
> +	 * However, if we do not walk the lists, we do not have to do
> +	 * SRCU because we have no references to any objects and do not
> +	 * need SRCU to keep them "alive".
> +	 */
> +	if (hlist_empty(&to_tell->i_fsnotify_marks) &&
> +	    (!mnt || hlist_empty(&mnt->mnt_fsnotify_marks)))
> +		return 0;
> +	/*
>  	 * if this is a modify event we may need to clear the ignored masks
>  	 * otherwise return if neither the inode nor the vfsmount care about
>  	 * this type of event.
> _
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR