x86: Put hot per CPU variables into a struct

https://lore.kernel.org/lkml/166601845607.401.14987443239536693764.tip-bot2@tip-bot2/T/

several patches changed the layout of “hot” percpu data.

e.g.

e57ef2ed Thomas Gleixner
x86: Put hot per CPU variables into a struct

64701838 Thomas Gleixner
x86/percpu: Move preempt_count next to current_task 

.. importantly:

commit e57ef2ed97c1d078973298658a8096644a1e9e09
Author: Thomas Gleixner <tglx@linutronix.de>
Date:   Thu Sep 15 13:11:01 2022 +0200

    x86: Put hot per CPU variables into a struct
    
    The layout of per-cpu variables is at the mercy of the compiler. This
    can lead to random performance fluctuations from build to build.
    
    Create a structure to hold some of the hottest per-cpu variables,
    starting with current_task.

what it does

some hot per cpu stuffs are defined from multiple places

// arch/x86/include/asm/preempt.h
DECLARE_PER_CPU(int, __preempt_count);

// arch/x86/include/asm/current.h
DECLARE_PER_CPU(struct task_struct *, current_task);

// ..

they are packed into struct pcpu_hot:

// arch/x86/include/asm/current.h

struct pcpu_hot {
	union {
		struct {
			struct task_struct	*current_task;
			int			preempt_count;
			int			cpu_number;
#ifdef CONFIG_MITIGATION_CALL_DEPTH_TRACKING
			u64			call_depth;
#endif
			unsigned long		top_of_stack;
			void			*hardirq_stack_ptr;
			u16			softirq_pending;
#ifdef CONFIG_X86_64
			bool			hardirq_stack_inuse;
#else
			void			*softirq_stack_ptr;
#endif
		};
		u8	pad[64];
	};
};

DECLARE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot);

/* const-qualified alias to pcpu_hot, aliased by linker. */
DECLARE_PER_CPU_ALIGNED(const struct pcpu_hot __percpu_seg_override,
			const_pcpu_hot);

and

// arch/x86/kernel/cpu/common.c
DEFINE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot) = {
	.current_task	= &init_task,
	.preempt_count	= INIT_PREEMPT_COUNT,
	.top_of_stack	= TOP_OF_INIT_STACK,
};
EXPORT_PER_CPU_SYMBOL(pcpu_hot);
EXPORT_PER_CPU_SYMBOL(const_pcpu_hot);

access, through the pcpu struct

-	per_cpu(current_task, cpu) = idle;
+	per_cpu(pcpu_hot.current_task, cpu) = idle;

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