[tip:x86/boot] x86/smpboot: Remove SIPI delays from cpu_up()

[tip-bot for Len Brown]

Commit-ID:  a9bcaa02a5104ace6a9d9e4a9cd9192a9e7744d6
Gitweb:     http://git.kernel.org/tip/a9bcaa02a5104ace6a9d9e4a9cd9192a9e7744d6
Author:     Len Brown <len.brown@xxxxxxxxx>
AuthorDate: Sun, 16 Aug 2015 11:45:47 -0400
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 17 Aug 2015 10:42:27 +0200

x86/smpboot: Remove SIPI delays from cpu_up()

MPS 1.4 example code shows the following required delays during processor
on-lining:

	INIT
	 udelay(10,000)
	SIPI
	 udelay(200)
	SIPI
	 udelay(200) /* Linux actually implements this as udelay(300) */

Linux skips the udelay(10,000) on modern processors.
This patch removes the udelay(200) after each SIPI
on those same processors.

All three legacy delays can be restored by the cmdline
"cpu_init_udelay=10000".

As measured by analyze_suspend.py, this patch speeds
processor resume time on my desktop from 2.4ms to 1.8ms, per AP.

Signed-off-by: Len Brown <len.brown@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Igor Mammedov <imammedo@xxxxxxxxxx>
Cc: Jan H. SchÃnherr <jschoenh@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Zhu Guihua <zhugh.fnst@xxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/a5dfdbc8fbfdd813784da204aad5677fe459ac37.1439739165.git.len.brown@xxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/kernel/smpboot.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 310b6f0..6740264 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -665,7 +665,8 @@ wakeup_secondary_cpu_via_init(int phys_apicid, unsigned long start_eip)
 		/*
 		 * Give the other CPU some time to accept the IPI.
 		 */
-		udelay(300);
+		if (init_udelay)
+			udelay(300);
 
 		pr_debug("Startup point 1\n");
 
@@ -675,7 +676,8 @@ wakeup_secondary_cpu_via_init(int phys_apicid, unsigned long start_eip)
 		/*
 		 * Give the other CPU some time to accept the IPI.
 		 */
-		udelay(200);
+		if (init_udelay)
+			udelay(200);
 
 		if (maxlvt > 3)		/* Due to the Pentium erratum 3AP.  */
 			apic_write(APIC_ESR, 0);
--
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