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Message-Id: <0aade12eabeb89a688c929fe80856eaea0544bb7.1439739165.git.len.brown@intel.com>
Date: Sun, 16 Aug 2015 11:45:46 -0400
From: Len Brown <lenb@...nel.org>
To: x86@...nel.org, linux-pm@...r.kernel.org,
linux-kernel@...r.kernel.org
Cc: Len Brown <len.brown@...el.com>
Subject: [PATCH 2/4] x86 smpboot: remove udelay(100) when polling cpu_callin_map
From: Len Brown <len.brown@...el.com>
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 <len.brown@...el.com>
---
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);
schedule();
}
}
--
2.5.0.330.g130be8e
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