linux-kernel - [PATCH 0/5] kernel/cpu.c: eliminate some indirection

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Date:	Fri, 25 Sep 2015 20:22:22 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 0/5] kernel/cpu.c: eliminate some indirection

Maybe third time's the charm...

The four cpumasks cpu_{possible,online,present,active}_bits are
exposed readonly via the corresponding const variables
cpu_xyz_mask. But they are also accessible for arbitrary writing via
the exposed functions set_cpu_xyz. There's quite a bit of code
throughout the kernel which iterates over or otherwise accesses these
bitmaps, and having the access go via the cpu_xyz_mask variables is
simply a useless indirection.

It may be that any problem in CS can be solved by an extra level of
indirection, but that doesn't mean every extra indirection solves a
problem. In this case, it even necessitates some minor ugliness (see
3/5).

The first four patches eliminate the cpu_xyz_mask variables by simply
exposing the actual bitmaps, after renaming them to discourage direct
access - that still happens through cpu_xyz_mask, which are now simply
macros with the same type and value as they used to have.

After that, there's no longer any reason to have the setter functions
be out-of-line: The boolean parameter is almost always a literal true
or false, so by making them static inlines they will usually compile
to one or two instructions.

For a defconfig build, bloat-o-meter says we save ~3000 bytes.

Rasmus Villemoes (5):
  kernel/cpu.c: change type of cpu_possible_bits and friends
  kernel/cpu.c: export __cpu_*_mask
  drivers/base/cpu.c: use __cpu_*_mask directly
  kernel/cpu.c: eliminate cpu_*_mask
  kernel/cpu.c: make set_cpu_* static inlines

 drivers/base/cpu.c      | 10 ++++----
 include/linux/cpumask.h | 55 +++++++++++++++++++++++++++++++++++-------
 kernel/cpu.c            | 64 ++++++++++---------------------------------------
 3 files changed, 65 insertions(+), 64 deletions(-)

-- 
2.1.3

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