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    <p>Normal global variables are not per CPU. Automatic variables are on the stack, and different CPUs use different stack, so naturally they get separate variables.</p> <p>I guess you're referring to Linux's per-CPU variable infrastructure.<br> Most of the magic is here (<code>asm-generic/percpu.h</code>):</p> <pre><code>extern unsigned long __per_cpu_offset[NR_CPUS]; #define per_cpu_offset(x) (__per_cpu_offset[x]) /* Separate out the type, so (int[3], foo) works. */ #define DEFINE_PER_CPU(type, name) \ __attribute__((__section__(".data.percpu"))) __typeof__(type) per_cpu__##name /* var is in discarded region: offset to particular copy we want */ #define per_cpu(var, cpu) (*RELOC_HIDE(&amp;per_cpu__##var, __per_cpu_offset[cpu])) #define __get_cpu_var(var) per_cpu(var, smp_processor_id()) </code></pre> <p>The macro <code>RELOC_HIDE(ptr, offset)</code> simply advances <code>ptr</code> by the given offset in bytes (regardless of the pointer type).</p> <p>What does it do?</p> <ol> <li>When defining <code>DEFINE_PER_CPU(int, x)</code>, an integer <code>__per_cpu_x</code> is created in the special <code>.data.percpu</code> section.</li> <li>When the kernel is loaded, this section is loaded multiple times - once per CPU (this part of the magic isn't in the code above).</li> <li>The <code>__per_cpu_offset</code> array is filled with the distances between the copies. Supposing 1000 bytes of per cpu data are used, <code>__per_cpu_offset[n]</code> would contain <code>1000*n</code>.</li> <li>The symbol <code>per_cpu__x</code> will be relocated, during load, to CPU 0's <code>per_cpu__x</code>.</li> <li><code>__get_cpu_var(x)</code>, when running on CPU 3, will translate to <code>*RELOC_HIDE(&amp;per_cpu__x, __per_cpu_offset[3])</code>. This starts with CPU 0's <code>x</code>, adds the offset between CPU 0's data and CPU 3's, and eventually dereferences the resulting pointer.</li> </ol>
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    1. POHow are percpu pointers implemented in the Linux kernel?
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    1. POHow are percpu pointers implemented in the Linux kernel?
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    1. COThanks for your ansser, but I still have some questions, new to smp, so, no offence to your idea. First, I thought the same process should have the same stack, here is thread definition in POSIX "...and automatic variables, are accessible to all threads in the same process.". Automatic variable are shared by threads. Different processor might have different stack segment register, but the content should be the same. Second, can we say that we can also access other cpu's variable if we want, just roll back the offset obtained by percpu?
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    2. COWhen two threads call function `foo`, which has an automatic variable `x`, there are two stacks and two instances of `x`. Each has a different address, and both threads can access both, if they have the address. With Linux's per-cpu variables, `per_cpu(var, cpu)` lets you access any cpu's variables.
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    3. COHow does the .data.percpu section determine whether the percpu variable is declared on the stack or heap?
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