[lkml]   [2015]   [Aug]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[tip:x86/boot] x86/smpboot: Remove udelay(100) when polling cpu_callin_map
Commit-ID:  2d99af8e8fd6c2dea11ab539f7aba69c37b845b4
Author: Len Brown <>
AuthorDate: Sun, 16 Aug 2015 11:45:46 -0400
Committer: Ingo Molnar <>
CommitDate: Mon, 17 Aug 2015 10:42:27 +0200

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 <>
Cc: Andy Lutomirski <>
Cc: Arjan van de Ven <>
Cc: Boris Ostrovsky <>
Cc: Borislav Petkov <>
Cc: Dave Hansen <>
Cc: Igor Mammedov <>
Cc: Jan H. Schönherr <>
Cc: Linus Torvalds <>
Cc: Paul E. McKenney <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Cc: Zhu Guihua <>
Signed-off-by: Ingo Molnar <>
arch/x86/kernel/smpboot.c | 1 -
1 file changed, 1 deletion(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 9ad88fb..310b6f0 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/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);

 \ /
  Last update: 2015-08-17 18:41    [W:0.057 / U:20.416 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site