linux-imx

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 <len.brown@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Jan H. Schönherr <jschoenh@amazon.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Zhu Guihua <zhugh.fnst@cn.fujitsu.com>
Link: http://lkml.kernel.org/r/0aade12eabeb89a688c929fe80856eaea0544bb7.1439739165.git.len.brown@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>