[PATCH] UefiCpuPkg/PiSmmCpuDxeSmm: Improve the performance of GetFreeToken()

[Ray Ni <niruiyu@users.noreply.github.com>]

Today's GetFreeToken() runs at the algorithm complexity of O(n)
where n is the size of the token list.

The change introduces a new global variable FirstFreeToken and it
always points to the first free token. So the algorithm complexity
of GetFreeToken() decreases from O(n) to O(1).

The improvement matters when some SMI code uses StartupThisAP()
service for each of the AP such that the algorithm complexity
becomes O(n) * O(m) where m is the AP count.

As next steps,
1. PROCEDURE_TOKEN.Used field can be optimized out because
all tokens before FirstFreeToken should have "Used" set while all
after FirstFreeToken should have "Used" cleared.
2. ResetTokens() can be optimized to only reset tokens before
FirstFreeToken.

Signed-off-by: Ray Ni <ray.ni@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
---
 UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c      | 71 ++++++++--------------
 UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h |  3 +-
 2 files changed, 27 insertions(+), 47 deletions(-)

diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
index c285a70ebb..7a223410d7 100644
--- a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
+++ b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
@@ -1,7 +1,7 @@
 /** @file
 SMM MP service implementation
 
-Copyright (c) 2009 - 2020, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2009 - 2021, Intel Corporation. All rights reserved.<BR>
 Copyright (c) 2017, AMD Incorporated. All rights reserved.<BR>
 
 SPDX-License-Identifier: BSD-2-Clause-Patent
@@ -453,6 +453,11 @@ ResetTokens (
 
     Link = GetNextNode (&gSmmCpuPrivate->TokenList, Link);
   }
+
+  //
+  // Reset the FirstFreeToken to the beginning of token list upon exiting SMI.
+  //
+  gSmmCpuPrivate->FirstFreeToken = GetFirstNode (&gSmmCpuPrivate->TokenList);
 }
 
 /**
@@ -1060,23 +1065,21 @@ IsTokenInUse (
 /**
   Allocate buffer for the SPIN_LOCK and PROCEDURE_TOKEN.
 
+  @return First token of the token buffer.
 **/
-VOID
+LIST_ENTRY *
 AllocateTokenBuffer (
   VOID
   )
 {
   UINTN               SpinLockSize;
   UINT32              TokenCountPerChunk;
-  UINTN               ProcTokenSize;
   UINTN               Index;
-  PROCEDURE_TOKEN     *ProcToken;
   SPIN_LOCK           *SpinLock;
   UINT8               *SpinLockBuffer;
-  UINT8               *ProcTokenBuffer;
+  PROCEDURE_TOKEN     *ProcTokens;
 
   SpinLockSize = GetSpinLockProperties ();
-  ProcTokenSize = sizeof (PROCEDURE_TOKEN);
 
   TokenCountPerChunk = FixedPcdGet32 (PcdCpuSmmMpTokenCountPerChunk);
   ASSERT (TokenCountPerChunk != 0);
@@ -1092,49 +1095,22 @@ AllocateTokenBuffer (
   SpinLockBuffer = AllocatePool (SpinLockSize * TokenCountPerChunk);
   ASSERT (SpinLockBuffer != NULL);
 
-  ProcTokenBuffer = AllocatePool (ProcTokenSize * TokenCountPerChunk);
-  ASSERT (ProcTokenBuffer != NULL);
+  ProcTokens = AllocatePool (sizeof (PROCEDURE_TOKEN) * TokenCountPerChunk);
+  ASSERT (ProcTokens != NULL);
 
   for (Index = 0; Index < TokenCountPerChunk; Index++) {
     SpinLock = (SPIN_LOCK *)(SpinLockBuffer + SpinLockSize * Index);
     InitializeSpinLock (SpinLock);
 
-    ProcToken = (PROCEDURE_TOKEN *)(ProcTokenBuffer + ProcTokenSize * Index);
-    ProcToken->Signature = PROCEDURE_TOKEN_SIGNATURE;
-    ProcToken->SpinLock = SpinLock;
-    ProcToken->Used = FALSE;
-    ProcToken->RunningApCount = 0;
+    ProcTokens[Index].Signature      = PROCEDURE_TOKEN_SIGNATURE;
+    ProcTokens[Index].SpinLock       = SpinLock;
+    ProcTokens[Index].Used           = FALSE;
+    ProcTokens[Index].RunningApCount = 0;
 
-    InsertTailList (&gSmmCpuPrivate->TokenList, &ProcToken->Link);
+    InsertTailList (&gSmmCpuPrivate->TokenList, &ProcTokens[Index].Link);
   }
-}
 
-/**
-  Find first free token in the allocated token list.
-
-  @retval    return the first free PROCEDURE_TOKEN.
-
-**/
-PROCEDURE_TOKEN *
-FindFirstFreeToken (
-  VOID
-  )
-{
-  LIST_ENTRY        *Link;
-  PROCEDURE_TOKEN   *ProcToken;
-
-  Link = GetFirstNode (&gSmmCpuPrivate->TokenList);
-  while (!IsNull (&gSmmCpuPrivate->TokenList, Link)) {
-    ProcToken = PROCEDURE_TOKEN_FROM_LINK (Link);
-
-    if (!ProcToken->Used) {
-      return ProcToken;
-    }
-
-    Link = GetNextNode (&gSmmCpuPrivate->TokenList, Link);
-  }
-
-  return NULL;
+  return &ProcTokens[0].Link;
 }
 
 /**
@@ -1154,12 +1130,15 @@ GetFreeToken (
 {
   PROCEDURE_TOKEN  *NewToken;
 
-  NewToken = FindFirstFreeToken ();
-  if (NewToken == NULL) {
-    AllocateTokenBuffer ();
-    NewToken = FindFirstFreeToken ();
+  //
+  // If FirstFreeToken meets the end of token list, enlarge the token list.
+  // Set FirstFreeToken to the first free token.
+  //
+  if (gSmmCpuPrivate->FirstFreeToken == &gSmmCpuPrivate->TokenList) {
+    gSmmCpuPrivate->FirstFreeToken = AllocateTokenBuffer ();
   }
-  ASSERT (NewToken != NULL);
+  NewToken = PROCEDURE_TOKEN_FROM_LINK (gSmmCpuPrivate->FirstFreeToken);
+  gSmmCpuPrivate->FirstFreeToken = GetNextNode (&gSmmCpuPrivate->TokenList, gSmmCpuPrivate->FirstFreeToken);
 
   NewToken->Used = TRUE;
   NewToken->RunningApCount = RunningApsCount;
diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h b/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h
index fe7e8b0323..17f4bd34fa 100644
--- a/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h
+++ b/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.h
@@ -1,7 +1,7 @@
 /** @file
 Agent Module to load other modules to deploy SMM Entry Vector for X86 CPU.
 
-Copyright (c) 2009 - 2019, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2009 - 2021, Intel Corporation. All rights reserved.<BR>
 Copyright (c) 2017, AMD Incorporated. All rights reserved.<BR>
 
 SPDX-License-Identifier: BSD-2-Clause-Patent
@@ -255,6 +255,7 @@ typedef struct {
 
   PROCEDURE_WRAPPER               *ApWrapperFunc;
   LIST_ENTRY                      TokenList;
+  LIST_ENTRY                      *FirstFreeToken;
 } SMM_CPU_PRIVATE_DATA;
 
 extern SMM_CPU_PRIVATE_DATA  *gSmmCpuPrivate;
-- 
2.21.0.windows.1