[tip:x86/urgent] x86/smpboot: Fix CPU #1 boot timeout

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

Commit-ID:  fcafddec4e78a7776db4b6685db6b2902d4300fc
Gitweb:     http://git.kernel.org/tip/fcafddec4e78a7776db4b6685db6b2902d4300fc
Author:     Len Brown <len.brown@intel.com>
AuthorDate: Fri, 16 Oct 2015 00:14:29 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 19 Oct 2015 09:14:41 +0200

x86/smpboot: Fix CPU #1 boot timeout

The following commit:

  a9bcaa02a5104ac ("x86/smpboot: Remove SIPI delays from cpu_up()")

Caused some Intel Core2 processors to time-out when bringing up CPU #1,
resulting in the missing of that CPU after bootup.

That patch reduced the SIPI delays from udelay() 300, 200 to udelay() 0,
0 on modern processors.

Several Intel(R) Core(TM)2 systems failed to bring up CPU #1 10/10 times
after that change.

Increasing either of the SIPI delays to udelay(1) results in
success. So here we increase both to udelay(10).  While this may
be 20x slower than the absolute minimum, it is still 20x to 30x
faster than the original code.

Tested-by: Donald Parsons <dparsons@brightdsl.net>
Tested-by: Shane <shrybman@teksavvy.com>
Signed-off-by: Len Brown <len.brown@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dparsons@brightdsl.net
Cc: shrybman@teksavvy.com
Link: http://lkml.kernel.org/r/6dd554ee8945984d85aafb2ad35793174d068af0.1444968087.git.len.brown@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/kernel/smpboot.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 32267cc..892ee2e5 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -660,7 +660,9 @@ wakeup_secondary_cpu_via_init(int phys_apicid, unsigned long start_eip)
 		/*
 		 * Give the other CPU some time to accept the IPI.
 		 */
-		if (init_udelay)
+		if (init_udelay == 0)
+			udelay(10);
+		else
 			udelay(300);
 
 		pr_debug("Startup point 1\n");
@@ -671,7 +673,9 @@ wakeup_secondary_cpu_via_init(int phys_apicid, unsigned long start_eip)
 		/*
 		 * Give the other CPU some time to accept the IPI.
 		 */
-		if (init_udelay)
+		if (init_udelay == 0)
+			udelay(10);
+		else
 			udelay(200);
 
 		if (maxlvt > 3)		/* Due to the Pentium erratum 3AP.  */