[PATCH v2 4/5] lib/cpumask: deprecate nr_cpumask_bits

[Yury Norov <yury.norov@gmail.com>]

Cpumask code is written in assumption that when CONFIG_CPUMASK_OFFSTACK
is enabled, all cpumasks have boot-time defined size, otherwise the size
is always NR_CPUS.

The latter is wrong because the number of possible cpus is always
calculated on boot, and it may be less than NR_CPUS.

On my 4-cpu arm64 VM the nr_cpu_ids is 4, as expected, and nr_cpumask_bits
is 256, which corresponds to NR_CPUS. This not only leads to useless
traversing of cpumask bits greater than 4, this also makes some cpumask
routines fail.

For example, cpumask_full(0b1111000..000) would erroneously return false
in the example above because tail bits in the mask are all unset.

This patch deprecates nr_cpumask_bits and wires it to nr_cpu_ids
unconditionally, so that cpumask routines will not waste time traversing
unused part of cpu masks. It also fixes cpumask_full() and similar
routines.

As a side effect, because now a length of cpumasks is defined at run-time
even if CPUMASK_OFFSTACK is disabled, compiler can't optimize corresponding
functions.

It increases kernel size by ~2.5KB if OFFSTACK is off. This is addressed in
the following patch.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
 include/linux/cpumask.h | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 8bac1dee8448..e01fba8ecc27 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -46,13 +46,8 @@ static inline void set_nr_cpu_ids(unsigned int nr)
 }
 #endif
 
-#ifdef CONFIG_CPUMASK_OFFSTACK
-/* Assuming NR_CPUS is huge, a runtime limit is more efficient.  Also,
- * not all bits may be allocated. */
+/* Deprecated. Always use nr_cpu_ids. */
 #define nr_cpumask_bits	nr_cpu_ids
-#else
-#define nr_cpumask_bits	((unsigned int)NR_CPUS)
-#endif
 
 /*
  * The following particular system cpumasks and operations manage
-- 
2.34.1