linux-kernel - Re: [PATCH 10/25] Unionfs: add un/likely conditionals on copyup ops

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Date:	Tue, 25 Sep 2007 21:32:44 -0700
From:	"Kok, Auke" <auke-jan.h.kok@...el.com>
To:	Erez Zadok <ezk@...sunysb.edu>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, viro@....linux.org.uk,
	hch@...radead.org
Subject: Re: [PATCH 10/25] Unionfs: add un/likely conditionals on copyup ops

Erez Zadok wrote:
> Signed-off-by: Erez Zadok <ezk@...sunysb.edu>
> ---
>  fs/unionfs/copyup.c |  102 +++++++++++++++++++++++++-------------------------
>  1 files changed, 51 insertions(+), 51 deletions(-)
> 
> diff --git a/fs/unionfs/copyup.c b/fs/unionfs/copyup.c
> index 23ac4c8..e3c5f15 100644
> --- a/fs/unionfs/copyup.c
> +++ b/fs/unionfs/copyup.c
> @@ -36,14 +36,14 @@ static int copyup_xattrs(struct dentry *old_lower_dentry,
>  
>  	/* query the actual size of the xattr list */
>  	list_size = vfs_listxattr(old_lower_dentry, NULL, 0);
> -	if (list_size <= 0) {
> +	if (unlikely(list_size <= 0)) {


I've been told several times that adding these is almost always bogus - either it
messes up the CPU branch prediction or the compiler/CPU just does a lot better at
finding the right way without these hints.

Adding them as a blanket seems rather strange. Have you got any numbers that this
really improves performance?

Auke
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