linux-kernel - [PATCH v2 4/5] lib/cpumask: deprecate nr_cpumask

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Date:   Mon,  5 Sep 2022 16:08:19 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Sander Vanheule <sander@...nheule.net>,
        Alexey Klimov <klimov.linux@...il.com>,
        Eric Biggers <ebiggers@...gle.com>
Subject: [PATCH v2 4/5] lib/cpumask: deprecate nr_cpumask_bits

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@...il.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