On Fri, Jun 19, 2015 at 02:50:25PM -0700, Dave Hansen wrote:
>
> From: Dave Hansen <[email protected]>
>
> 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 <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Jan Kara <[email protected]>
> Cc: Al Viro <[email protected]>
> Cc: Eric Paris <[email protected]>
> Cc: John McCutchan <[email protected]>
> Cc: Robert Love <[email protected]>
> Cc: Tim Chen <[email protected]>
> Cc: Andi Kleen <[email protected]>
> Cc: [email protected]
> ---
>
> 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);
>
> _
--
[email protected] -- Speaking for myself only
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