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

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Date:   Wed, 25 Jan 2017 16:15:19 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Linux Kernel Mailing List <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 4:09 PM, Christoph Hellwig <hch@...radead.org> wrote:
> 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.

That would defines help, thanks!

I also noticed that my patch wouldn't work, as unlikely_notrace() is not
defined unless we are actually tracing, so while it fixes some rare
configurations, it breaks all the configurations that matter.

    Arnd