linux-kernel - [PATCH v4 4/5] cpumask: Add UP optimised for_each_*

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Date:   Sat,  2 Jul 2022 18:08:27 +0200
From:   Sander Vanheule <sander@...nheule.net>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        elver@...gle.com, gregkh@...uxfoundation.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, vschneid@...hat.com,
        Yury Norov <yury.norov@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Sander Vanheule <sander@...nheule.net>
Subject: [PATCH v4 4/5] cpumask: Add UP optimised for_each_*_cpu versions

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 7fbef41b3093..6c5b4ee000f2 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -750,9 +750,16 @@ 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