linux-kernel - [PATCH v1 2/2] cpumask: Add UP optimised for_each_*

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Message-Id: <a4986bd3a692a35953111a5ad20a2ec55be8670a.1654201862.git.sander@svanheule.net>
Date:   Thu,  2 Jun 2022 23:04:20 +0200
From:   Sander Vanheule <sander@...nheule.net>
To:     Peter Zijlstra <peterz@...radead.org>,
        Yury Norov <yury.norov@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Marco Elver <elver@...gle.com>,
        Barry Song <song.bao.hua@...ilicon.com>
Cc:     linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Sander Vanheule <sander@...nheule.net>
Subject: [PATCH v1 2/2] cpumask: Add UP optimised for_each_*_cpu loops

On uniprocessor builds, the following loops will always run over a mask
that contains one enabled CPU (cpu0):
    - for_each_possible_cpu
    - for_each_online_cpu
    - for_each_present_cpu

Provide uniprocessor-specific macros for these loops, that always run
exactly once.

Signed-off-by: Sander Vanheule <sander@...nheule.net>
---
 include/linux/cpumask.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index ce8c7b27f6c9..8af37cd603e3 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -806,9 +806,16 @@ extern const DECLARE_BITMAP(cpu_all_bits, NR_CPUS);
 /* First bits of cpu_bit_bitmap are in fact unset. */
 #define cpu_none_mask to_cpumask(cpu_bit_bitmap[0])
 
+#if NR_CPUS == 1
+/* Uniprocessor: the possible/online/present masks are always "1" */
+#define for_each_possible_cpu(cpu)	for ((cpu) = 0; (cpu) < 1; (cpu)++)
+#define for_each_online_cpu(cpu)	for ((cpu) = 0; (cpu) < 1; (cpu)++)
+#define for_each_present_cpu(cpu)	for ((cpu) = 0; (cpu) < 1; (cpu)++)
+#else
 #define for_each_possible_cpu(cpu) for_each_cpu((cpu), cpu_possible_mask)
 #define for_each_online_cpu(cpu)   for_each_cpu((cpu), cpu_online_mask)
 #define for_each_present_cpu(cpu)  for_each_cpu((cpu), cpu_present_mask)
+#endif
 
 /* Wrappers for arch boot code to manipulate normally-constant masks */
 void init_cpu_present(const struct cpumask *src);
-- 
2.36.1