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Subject[PATCH 2/4] x86 smpboot: remove udelay(100) when polling cpu_callin_map
From: Len Brown <>

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 <>
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);

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