kernel/smp: add a Kconfig to disable validation of _current_cpu

The assert validation was added in eefd3da to ensure that
`_current_cpu` is used safely. However, since `_current_cpu`
is used frequently in the kernel/scheduler, especially when
SMP is enabled, the assertion has a negative impact on the
performance.

This commit introduces a Kconfig to disable just the assert
validation of the `_current_cpu`, without having to disable
`CONFIG_ASSERT` entirely.

Signed-off-by: Yong Cong Sin <[email protected]>

Author: ycsin

include/zephyr/kernel_structs.h

@@ -258,8 +258,11 @@ extern atomic_t _cpus_active;
  */
 bool z_smp_cpu_mobile(void);
 
-#define _current_cpu ({ __ASSERT_NO_MSG(!z_smp_cpu_mobile()); \
-			arch_curr_cpu(); })
+#define _current_cpu                                                                               \
+	({                                                                                         \
+		IF_ENABLED(CONFIG_CURRENT_CPU_VALIDATE, (__ASSERT_NO_MSG(!z_smp_cpu_mobile())));   \
+		arch_curr_cpu();                                                                   \
+	})
 #define _current k_sched_current_thread_query()
 
 #else

kernel/Kconfig.smp

@@ -126,4 +126,12 @@ config TICKET_SPINLOCKS
 	  which resolves such unfairness issue at the cost of slightly
 	  increased memory footprint.
 
+config CURRENT_CPU_VALIDATE
+	bool "Validate usage of _current_cpu"
+	depends on SMP && ASSERT
+	default y if MP_MAX_NUM_CPUS > 1
+	help
+	  The `_current_cpu` pointer should only ever be used in non-preemptible
+	  contexts. When assertions are enabled in SMP systems, this option guarantees
+	  correct `_current_cpu` usage with runtime checks.
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