How can I explain for this? what is __kernel_vsyscall () ?

[Tom Frank]

I debug a multithreads  programme  with gdb,as it runs  I  use the
'info threads' command, and find that  most of the threads running at
__kernel_vsyscall () function

as below:

  6    Thread 0xb61e4b90 (LWP 322) 0x008e7402 in __kernel_vsyscall ()
  5    Thread 0xb6be5b90 (LWP 320) 0x008e7402 in __kernel_vsyscall ()
  4    Thread 0xb7fe7b90 (LWP 318) 0x008e7402 in __kernel_vsyscall ()
  3    Thread 0xb75e6b90 (LWP 316) 0x008e7402 in __kernel_vsyscall ()
* 1    Thread 0xb7fe86d0 (LWP 312) 0x0074c2b8 in clone () from
/lib/i686/nosegneg/libc.so.6



what is __kernel_vsyscall (),  why most threads's frame  is in that  step ?