[tip:x86/boot] x86/smpboot: Remove udelay(100) when polling cpu_initialized_map

[tip-bot for Len Brown <tipbot@zytor.com>]

Commit-ID:  6e38f1e79d16f4fa9e5cf06792500e11c96a6f84
Gitweb:     http://git.kernel.org/tip/6e38f1e79d16f4fa9e5cf06792500e11c96a6f84
Author:     Len Brown <len.brown@intel.com>
AuthorDate: Sun, 16 Aug 2015 11:45:45 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 17 Aug 2015 10:42:27 +0200

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

After the BSP sends the APIC INIT/SIPI/SIPI to the AP,
it waits for the AP to come up and indicate that it is alive
by setting its own bit in the cpu_initialized_mask.

Linux polls for up to 10 seconds for this to happen.
Each polling loop has a udelay(100) and a call to schedule().

The udelay(100) adds no value.

For example, on my desktop, the BSP waits for the
other 3 CPUs to come on line at boot for 305, 404, 405 usec.
For resume from S3, it waits 317, 404, 405 usec.

But when the udelay(100) is removed, the BSP waits
305, 310, 306 for boot, and 305, 307, 306 for resume.

So for both boot and resume, removing the udelay(100)
speeds online by about 100us in 2 of 3 cases.

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/33ef746c67d2489cad0a9b1958cf71167232ff2b.1439739165.git.len.brown@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/kernel/smpboot.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index b1f3ed9c..9ad88fb 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -898,7 +898,7 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
 
 	if (!boot_error) {
 		/*
-		 * Wait 10s total for a response from AP
+		 * Wait 10s total for first sign of life from AP
 		 */
 		boot_error = -1;
 		timeout = jiffies + 10*HZ;
@@ -911,7 +911,6 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
 				boot_error = 0;
 				break;
 			}
-			udelay(100);
 			schedule();
 		}
 	}