[RFC][PATCH v1] UefiCpuPkg/MpInitLib DXE: Reduce AP status check interval

["Wu, Hao A" <hao.a.wu@intel.com>]

This commit will reduce the interval of the AP status check event from 100
milliseconds to 10 milliseconds.

(I searched the history of the 100ms interval, it seems no comment or log
message was mentioned for the choice of this value. Looks like the value
is selected by experience.)

The purpose is to reduce the response time when the BSP calls below
EFI_MP_SERVICES_PROTOCOL services in a non-blocking manner:

* StartupAllAPs()
* StartupThisAP()

Reducing the check interval will benefit the performance for the case when
the BSP uses WaitForEvent() or uses CheckEvent() in a loop to wait for
AP(s) to complete the task, especially when the task can be finished
considerably fast on AP(s).

An example is within function CpuFeaturesInitialize() under
UefiCpuPkg/Library/RegisterCpuFeaturesLib/DxeRegisterCpuFeaturesLib.c,
where BSP will perform the same task with APs and requires all the
processors to finish the task before BSP proceeds to its next task.

Impact:
A. The impact is minimal when there is no non-blocking calls of the
   StartupAllAPs/StartupThisAp MP services, because the check function
   CheckAndUpdateApsStatus() will return directly when there is no
   registered wait event (i.e. no non-blocking request).

B. There will be a performance tradeoff when BSP continues to proceed
   other tasks after submitting a non-blocking StartupAllAPs/StartupThisAP
   request. If the AP status check takes a good portion of the shortened
   interval, BSP will have less time slice working on its own task before
   all the APs complete their tasks.

My investigation for Impact B is that it is a rare scenario in the edk2
code base.

Unitests:
A. OS boot successfully.
B. System (with 24 threads) boot time reduced. Almost all the saved time
   comes from the above-mentioned case in CpuFeaturesInitialize().

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Signed-off-by: Hao A Wu <hao.a.wu@intel.com>
---
 UefiCpuPkg/Library/MpInitLib/DxeMpLib.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
index a987c32109..9ba886e8ed 100644
--- a/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
+++ b/UefiCpuPkg/Library/MpInitLib/DxeMpLib.c
@@ -15,7 +15,7 @@
 
 #include <Protocol/Timer.h>
 
-#define  AP_CHECK_INTERVAL     (EFI_TIMER_PERIOD_MILLISECONDS (100))
+#define  AP_CHECK_INTERVAL     (EFI_TIMER_PERIOD_MILLISECONDS (10))
 #define  AP_SAFE_STACK_SIZE    128
 
 CPU_MP_DATA      *mCpuMpData = NULL;
-- 
2.12.0.windows.1