linux-kernel - Re: [PATCH] [RFC] xfs: work around unlikely() profiler glitch

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Date:   Wed, 25 Jan 2017 07:09:29 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Nicolas Pitre <nico@...aro.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org, Dave Chinner <dchinner@...hat.com>,
        Brian Foster <bfoster@...hat.com>,
        Eric Sandeen <sandeen@...deen.net>
Subject: Re: [PATCH] [RFC] xfs: work around unlikely() profiler glitch

On Wed, Jan 25, 2017 at 03:08:10PM +0100, Arnd Bergmann wrote:
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index d22f7930eb75..dca3ddd737d4 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3629,7 +3629,7 @@ xfs_bmap_btalloc(
>  		align = xfs_get_cowextsz_hint(ap->ip);
>  	else if (xfs_alloc_is_userdata(ap->datatype))
>  		align = xfs_get_extsz_hint(ap->ip);
> -	if (unlikely(align)) {
> +	if (unlikely_notrace(align)) {
>  		error = xfs_bmap_extsize_align(mp, &ap->got, &ap->prev,
>  						align, 0, ap->eof, 0, ap->conv,
>  						&ap->offset, &ap->length);

The unlikely calls on align in xfs_bmap_btalloc should simply be
removed.  They aren't actually unlikely for many workloads.  I have
a patch in my queue that I can expedite based on your report.