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

[Andi Kleen <ak@linux.intel.com>]

On Fri, Jun 19, 2015 at 02:50:25PM -0700, 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
> being modified *or* the vfsmount.

What is so expensive in it? Just the memory barrier in it?

Perhaps the function can be tuned in general.

-Andi

int __srcu_read_lock(struct srcu_struct *sp)
{
        int idx;

        idx = ACCESS_ONCE(sp->completed) & 0x1;
        preempt_disable();
        __this_cpu_inc(sp->per_cpu_ref->c[idx]);
        smp_mb(); /* B */  /* Avoid leaking the critical section. */
        __this_cpu_inc(sp->per_cpu_ref->seq[idx]);
        preempt_enable();
        return idx;
}


> 
> The *_fsnotify_mask is an aggregate of each of the masks from
> each mark.  If we have nothing set in the masks at all then there
> are no marks and no need to do anything with 'ignored masks'
> since none exist.  This keeps us from having to do the costly
> srcu_read_lock() for a check which is very cheap.
> 
> This patch gave a 10.8% speedup in writes/second on my test.
> 
> 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: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Andi Kleen <ak@linux.intel.com>
> Cc: linux-kernel@vger.kernel.org
> ---
> 
>  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	2015-06-19 13:29:53.117283581 -0700
> +++ b/fs/notify/fsnotify.c	2015-06-19 13:29:53.123283853 -0700
> @@ -213,6 +213,16 @@ int fsnotify(struct inode *to_tell, __u3
>  	    !(test_mask & to_tell->i_fsnotify_mask) &&
>  	    !(mnt && test_mask & mnt->mnt_fsnotify_mask))
>  		return 0;
> +	/*
> +	 * Optimization: The *_fsnotify_mask is an aggregate of each of the
> +	 * masks from each mark.  If we have nothing set in the masks at
> +	 * all then there are no marks and no need to do anything with
> +	 * 'ignored masks' since none exist.  This keeps us from having to
> +	 * do the costly srcu_read_lock() for a check which is very cheap.
> +	 */
> +	if (!to_tell->i_fsnotify_mask &&
> +	    (!mnt || !mnt->mnt_fsnotify_mask))
> +		return 0;
>  
>  	idx = srcu_read_lock(&fsnotify_mark_srcu);
>  
> _

-- 
ak@linux.intel.com -- Speaking for myself only
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