x86/smpboot: Remove udelay(100) when polling cpu_callin_map

After the BSP sends INIT/SIPI/SIP to the AP and sees the AP
in the cpu_initialized_map, it sets the AP loose via the
cpu_callout_map, and waits for it via the cpu_callin_map.

The BSP polls the cpu_callin_map with a udelay(100)
and a schedule() in each iteration.

The udelay(100) adds no value.

For example, on my 4-CPU dekstop, the AP finishes
cpu_callin() in under 70 usec and sets the cpu_callin_mask.
The BSP, however, doesn't see that setting until over 30 usec
later, because it was still running its udelay(100)
when the AP finished.

Deleting the udelay(100) in the cpu_callin_mask polling loop,
saves from 0 to 100 usec per Application Processor.

Signed-off-by: Len Brown <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Igor Mammedov <[email protected]>
Cc: Jan H. Schönherr <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Zhu Guihua <[email protected]>
Link: http://lkml.kernel.org/r/0aade12eabeb89a688c929fe80856eaea0544bb7.1439739165.git.len.brown@intel.com
Signed-off-by: Ingo Molnar <[email protected]>

arch/x86/kernel/smpboot.c

@@ -926,7 +926,6 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
 			 * for the MTRR work(triggered by the AP coming online)
 			 * to be completed in the stop machine context.
 			 */
-			udelay(100);
 			schedule();
 		}
 	}