linux-kernel - Re: [PATCH v2] consolidate WARN

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Date:	Tue, 28 Feb 2012 08:58:58 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Andrew Morton" <akpm@...ux-foundation.org>
Cc:	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] consolidate WARN_...ONCE() static variables

>>> On 28.02.12 at 09:32, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Tue, 28 Feb 2012 08:16:40 +0000 "Jan Beulich" <JBeulich@...e.com> wrote:
> 
>> Oh, sorry - to carry static data the accesses to which are unlikely
>> (i.e., as in the case given, fully contained in code sections inside
>> conditionals which themselves use unlikely() on their primary/only
>> clause - in other words, something that the compiler really could
>> do on its own).
> 
> I think I just learned more about this patch than at any time since we
> started discussing it.
> 
> Why add a new section, rather than using __read_mostly?

Because __read_mostly data frequently is hot.

> I suppose we should add and use a #define for this, like __read_mostly.
> That would be a good site for documenting it ;)

That's a possibility of course. I'll do so in an eventual v3.

> And I come back to my old friend printk_once().  If I'm understanding
> things correctly, we can/should make that test unlikely, then mark
> __print_once as __this_new_section?  Otherwise... help!

No. The variable itself serves for the tested condition here, and
hence a priori you can't say whether the whole construct sits in
a hot path (whether putting it in a hot path is a good idea is
another question, but the print-this-one-time-only nature of it
may be the very reason why someone considers this reasonable
for his code).

As opposed to that, the access to the static variable in WARN_ONCE()
is *after* an unlikely() condition was already evaluated, i.e. would
generally sit on a code path that we hope doesn't get speculated
along (often), by means of the compiler suitably arranging branch
targets.

> btw, I don't think there's a significant performance benefit here - if
> the kernel is ever executing WARN_ON_ONCE(), WARN_ONCE() or
> printk_once() with any frequency then it is already badly broken.

The intended performance benefit isn't with the WARN_ONCE()
constructs - we certainly don't care much about them being executed
efficiently - but instead with code path accessing other data in the
same cache line (or spilled across multiple cache lines just because
of the __warned variable sitting in the middle).

> Which brings us down to saving a bit of space.  And I don't think I see
> how this saves space?

The space saving results from grouping (many) 1-byte entities
together, which (when emitted normally) will generally require
padding to 4 or 8 bytes (as being adjacent with other static data
in the same or next compilation unit). That padding won't occur
if all of the items in a given section are of the same size (and
alignment).

Jan

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