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       getcpu - determine CPU and NUMA node on which the calling thread is running


       #include <linux/getcpu.h>

       int getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *tcache);


       The  getcpu() system call identifies the processor and node on which the calling thread or
       process is currently running and writes them into the integers pointed to by the  cpu  and
       node arguments.  The processor is a unique small integer identifying a CPU.  The node is a
       unique small identifier identifying a NUMA node.  When either cpu or node is NULL  nothing
       is written to the respective pointer.

       The third argument to this system call is nowadays unused, and should be specified as NULL
       unless portability to Linux 2.6.23 or earlier is required (see NOTES).

       The information placed in cpu is guaranteed to be current only at the time  of  the  call:
       unless the CPU affinity has been fixed using sched_setaffinity(2), the kernel might change
       the CPU at any time.  (Normally this does  not  happen  because  the  scheduler  tries  to
       minimize  movements between CPUs to keep caches hot, but it is possible.)  The caller must
       allow for the possibility that the information returned in  cpu  and  node  is  no  longer
       current by the time the call returns.


       On success, 0 is returned.  On error, -1 is returned, and errno is set appropriately.


       EFAULT Arguments point outside the calling process's address space.


       getcpu()  was  added  in  kernel 2.6.19 for x86-64 and i386.  Library support was added in
       glibc 2.29 (Earlier glibc versions did  not  provide  a  wrapper  for  this  system  call,
       necessitating the use of syscall(2).)


       getcpu() is Linux-specific.


       Linux  makes a best effort to make this call as fast as possible.  (On some architectures,
       this is done via an implementation in the vdso(7).)  The intention of getcpu() is to allow
       programs to make optimizations with per-CPU data or for NUMA optimization.

       The  tcache  argument  is unused since Linux 2.6.24.  In earlier kernels, if this argument
       was non-NULL, then it specified a pointer to a  caller-allocated  buffer  in  thread-local
       storage that was used to provide a caching mechanism for getcpu().  Use of the cache could
       speed getcpu() calls, at the cost that there was a very small  chance  that  the  returned
       information  would be out of date.  The caching mechanism was considered to cause problems
       when migrating threads between CPUs, and so the argument is now ignored.


       mbind(2), sched_setaffinity(2), set_mempolicy(2), sched_getcpu(3), cpuset(7), vdso(7)


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