01/01: [NTOS:KE] Don't loop indefinitely trying to figure out the CPU frequency. CORE-14419

[Thomas Faber]

https://git.reactos.org/?p=reactos.git;a=commitdiff;h=6b78ff036fb4f446f4e0a…

commit 6b78ff036fb4f446f4e0aebe5120af26d6183a77
Author:     Thomas Faber <thomas.faber(a)reactos.org>
AuthorDate: Thu Mar 1 10:36:27 2018 +0100
Commit:     Thomas Faber <thomas.faber(a)reactos.org>
CommitDate: Thu Mar 1 14:26:44 2018 +0100

    [NTOS:KE] Don't loop indefinitely trying to figure out the CPU frequency.
CORE-14419
    
    Previously, we would keep sampling the CPU frequency until two subsequent
    samples differed by at most 1 MHz. This could take several seconds, and would
    unnecessarily delay boot.
    Instead, if sampling is too unreliable, just give up and calculate the average
    frequency from 10 samples. This is no worse than picking the frequency that
    just happened to be returned twice in a row.
    
    The fact that this method of sampling fails could indicate that there's a
    problem with our performance counter implementation or timer interrupt,
    but that's a separate issue...
---
 ntoskrnl/ke/i386/kiinit.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/ntoskrnl/ke/i386/kiinit.c b/ntoskrnl/ke/i386/kiinit.c
index cba2c423fd..c17f4dd1f3 100644
--- a/ntoskrnl/ke/i386/kiinit.c
+++ b/ntoskrnl/ke/i386/kiinit.c
@@ -44,7 +44,7 @@ KiInitMachineDependent(VOID)
     PFX_SAVE_AREA FxSaveArea;
     ULONG MXCsrMask = 0xFFBF;
     CPU_INFO CpuInfo;
-    KI_SAMPLE_MAP Samples[4];
+    KI_SAMPLE_MAP Samples[10];
     PKI_SAMPLE_MAP CurrentSample = Samples;
     LARGE_IDENTITY_MAP IdentityMap;
 
@@ -240,11 +240,17 @@ KiInitMachineDependent(VOID)
                     CurrentSample++;
                     Sample++;
 
-                    if (Sample == sizeof(Samples) / sizeof(Samples[0]))
+                    if (Sample == RTL_NUMBER_OF(Samples))
                     {
-                        /* Restart */
-                        CurrentSample = Samples;
-                        Sample = 0;
+                        /* No luck. Average the samples and be done */
+                        ULONG TotalMHz = 0;
+                        while (Sample--)
+                        {
+                            TotalMHz += Samples[Sample].MHz;
+                        }
+                        CurrentSample[-1].MHz = TotalMHz / RTL_NUMBER_OF(Samples);
+                        DPRINT1("Sampling CPU frequency failed. Using average of %lu
MHz\n", CurrentSample[-1].MHz);
+                        break;
                     }
                 }