linux-kernel - Re: [PATCH] consolidate WARN

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Message-Id: <20120106030329.d934f5bc.akpm@linux-foundation.org>
Date:	Fri, 6 Jan 2012 03:03:29 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Jan Beulich" <JBeulich@...e.com>
Cc:	"Michal Marek" <mmarek@...e.cz>, <linux-kbuild@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] consolidate WARN_...ONCE() static variables

On Fri, 06 Jan 2012 07:40:29 +0000 "Jan Beulich" <JBeulich@...e.com> wrote:

> >>> On 05.01.12 at 22:03, Andrew Morton <akpm@...ux-foundation.org> wrote:
> > On Thu, 05 Jan 2012 11:09:40 +0000
> > "Jan Beulich" <JBeulich@...e.com> wrote:
> > 
> >> >>> On 05.01.12 at 00:03, Andrew Morton <akpm@...ux-foundation.org> wrote:
> >> > On Thu, 22 Dec 2011 15:53:49 +0000
> >> > "Jan Beulich" <JBeulich@...e.com> wrote:
> >> > 
> >> >> Due to the alignment of following variables, these typically consume
> >> >> more than just the single byte that 'bool' requires, and as there are
> >> >> a few hundred instances, the cache pollution (not so much the waste of
> >> >> memory) sums op. Put these variables into their own section, outside
> >> >> of half way frequently used memory range.
> >> >> 
> >>
> >> ...
> >>
> >> > printk_once() should also be converted.  And ata_print_version_once(),
> >> > if it insists on continuing to exist.
> >> 
> >> I disagree for those (and intentionally didn't touch printk_once();
> >> wasn't aware of the other) - at best this could get marked
> >> __read_mostly, but that's not the subject of this patch.
> > 
> > Confused.  It is exactly the subject of the patch?
> 
> No - the goal here is to eliminate the wasteful alignment holes
> created by the __warned variables in the WARN_...ONCE()
> instances.

What are these alignment holes?  I'd assumed (without thinking a lot)
that they were little three or two byte gaps because sizeof(bool)=1 or
2.  But I see that sizeof(bool) is actually 4, so I don't know what
you're talking about.

Apparently there is some gcc behaviour which you know about and I
don't.

> These get accessed past and unlikely() condition,
> and hence get moved into a separate data section (so they
> would all end up together, with no holes in between).
>
>
> ...
>
> > I'm suspecting that there is some changelog crappiness going on here. 
> > What didn't you tell us?
> 
> I think the original description says all that it has to.

If it did that, I wouldn't have had any questions to ask you.
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