1 files changed, 79 insertions, 26 deletions

diff --git a/src/base/spinlock.cc b/src/base/spinlock.cc
index 85ff21e..5ff9cf0 100644
--- a/src/base/spinlock.cc
+++ b/src/base/spinlock.cc
@@ -1,4 +1,3 @@
-// -*- Mode: C++; c-basic-offset: 2; indent-tabs-mode: nil -*-
 /* Copyright (c) 2006, Google Inc.
  * All rights reserved.
  * 
@@ -34,14 +33,21 @@
 
 #include <config.h>
 #include "base/spinlock.h"
+#include "base/synchronization_profiling.h"
 #include "base/spinlock_internal.h"
-#include "base/sysinfo.h"   /* for GetSystemCPUsCount() */
+#include "base/sysinfo.h"   /* for NumCPUs() */
+#include "base/cycleclock.h"
 
 // NOTE on the Lock-state values:
 //
-// kSpinLockFree represents the unlocked state
-// kSpinLockHeld represents the locked state with no waiters
-// kSpinLockSleeper represents the locked state with waiters
+//   kSpinLockFree represents the unlocked state
+//   kSpinLockHeld represents the locked state with no waiters
+//
+// Values greater than kSpinLockHeld represent the locked state with waiters,
+// where the value is the time the current lock holder had to
+// wait before obtaining the lock.  The kSpinLockSleeper state is a special
+// "locked with waiters" state that indicates that a sleeper needs to
+// be woken, but the thread that just released the lock didn't wait.
 
 static int adaptive_spin_count = 0;
 
@@ -53,7 +59,7 @@ struct SpinLock_InitHelper {
   SpinLock_InitHelper() {
     // On multi-cpu machines, spin for longer before yielding
     // the processor or sleeping.  Reduces idle time significantly.
-    if (GetSystemCPUsCount() > 1) {
+    if (NumCPUs() > 1) {
       adaptive_spin_count = 1000;
     }
   }
@@ -64,28 +70,35 @@ struct SpinLock_InitHelper {
 // but nothing lock-intensive should be going on at that time.
 static SpinLock_InitHelper init_helper;
 
-inline void SpinlockPause(void) {
-#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
-  __asm__ __volatile__("rep; nop" : : );
-#endif
-}
-
 }  // unnamed namespace
 
-// Monitor the lock to see if its value changes within some time
-// period (adaptive_spin_count loop iterations). The last value read
+// Monitor the lock to see if its value changes within some time period
+// (adaptive_spin_count loop iterations).  A timestamp indicating
+// when the thread initially started waiting for the lock is passed in via
+// the initial_wait_timestamp value.  The total wait time in cycles for the
+// lock is returned in the wait_cycles parameter.  The last value read
 // from the lock is returned from the method.
-Atomic32 SpinLock::SpinLoop() {
+Atomic32 SpinLock::SpinLoop(int64 initial_wait_timestamp,
+                            Atomic32* wait_cycles) {
   int c = adaptive_spin_count;
   while (base::subtle::NoBarrier_Load(&lockword_) != kSpinLockFree && --c > 0) {
-    SpinlockPause();
   }
-  return base::subtle::Acquire_CompareAndSwap(&lockword_, kSpinLockFree,
-                                              kSpinLockSleeper);
+  Atomic32 spin_loop_wait_cycles = CalculateWaitCycles(initial_wait_timestamp);
+  Atomic32 lock_value =
+      base::subtle::Acquire_CompareAndSwap(&lockword_, kSpinLockFree,
+                                           spin_loop_wait_cycles);
+  *wait_cycles = spin_loop_wait_cycles;
+  return lock_value;
 }
 
 void SpinLock::SlowLock() {
-  Atomic32 lock_value = SpinLoop();
+  // The lock was not obtained initially, so this thread needs to wait for
+  // it.  Record the current timestamp in the local variable wait_start_time
+  // so the total wait time can be stored in the lockword once this thread
+  // obtains the lock.
+  int64 wait_start_time = CycleClock::Now();
+  Atomic32 wait_cycles;
+  Atomic32 lock_value = SpinLoop(wait_start_time, &wait_cycles);
 
   int lock_wait_call_count = 0;
   while (lock_value != kSpinLockFree) {
@@ -100,16 +113,16 @@ void SpinLock::SlowLock() {
                                                         kSpinLockSleeper);
       if (lock_value == kSpinLockHeld) {
         // Successfully transitioned to kSpinLockSleeper.  Pass
-        // kSpinLockSleeper to the SpinLockDelay routine to properly indicate
+        // kSpinLockSleeper to the SpinLockWait routine to properly indicate
         // the last lock_value observed.
         lock_value = kSpinLockSleeper;
       } else if (lock_value == kSpinLockFree) {
-        // Lock is free again, so try and acquire it before sleeping.  The
+        // Lock is free again, so try and aquire it before sleeping.  The
         // new lock state will be the number of cycles this thread waited if
         // this thread obtains the lock.
         lock_value = base::subtle::Acquire_CompareAndSwap(&lockword_,
                                                           kSpinLockFree,
-                                                          kSpinLockSleeper);
+                                                          wait_cycles);
         continue;  // skip the delay at the end of the loop
       }
     }
@@ -119,11 +132,51 @@ void SpinLock::SlowLock() {
                                   ++lock_wait_call_count);
     // Spin again after returning from the wait routine to give this thread
     // some chance of obtaining the lock.
-    lock_value = SpinLoop();
+    lock_value = SpinLoop(wait_start_time, &wait_cycles);
+  }
+}
+
+// The wait time for contentionz lock profiling must fit into 32 bits.
+// However, the lower 32-bits of the cycle counter wrap around too quickly
+// with high frequency processors, so a right-shift by 7 is performed to
+// quickly divide the cycles by 128.  Using these 32 bits, reduces the
+// granularity of time measurement to 128 cycles, and loses track
+// of wait time for waits greater than 109 seconds on a 5 GHz machine
+// [(2^32 cycles/5 Ghz)*128 = 109.95 seconds]. Waits this long should be
+// very rare and the reduced granularity should not be an issue given
+// processors in the Google fleet operate at a minimum of one billion
+// cycles/sec.
+enum { PROFILE_TIMESTAMP_SHIFT = 7 };
+
+void SpinLock::SlowUnlock(uint64 wait_cycles) {
+  base::internal::SpinLockWake(&lockword_, false);  // wake waiter if necessary
+
+  // Collect contentionz profile info, expanding the wait_cycles back out to
+  // the full value.  If wait_cycles is <= kSpinLockSleeper, then no wait
+  // was actually performed, so don't record the wait time.  Note, that the
+  // CalculateWaitCycles method adds in kSpinLockSleeper cycles
+  // unconditionally to guarantee the wait time is not kSpinLockFree or
+  // kSpinLockHeld.  The adding in of these small number of cycles may
+  // overestimate the contention by a slight amount 50% of the time.  However,
+  // if this code tried to correct for that addition by subtracting out the
+  // kSpinLockSleeper amount that would underestimate the contention slightly
+  // 50% of the time.  Both ways get the wrong answer, so the code
+  // overestimates to be more conservative. Overestimating also makes the code
+  // a little simpler.
+  //
+  if (wait_cycles > kSpinLockSleeper) {
+    base::SubmitSpinLockProfileData(this,
+                                    wait_cycles << PROFILE_TIMESTAMP_SHIFT);
   }
 }
 
-void SpinLock::SlowUnlock() {
-  // wake waiter if necessary
-  base::internal::SpinLockWake(&lockword_, false);
+inline int32 SpinLock::CalculateWaitCycles(int64 wait_start_time) {
+  int32 wait_cycles = ((CycleClock::Now() - wait_start_time) >>
+                       PROFILE_TIMESTAMP_SHIFT);
+  // The number of cycles waiting for the lock is used as both the
+  // wait_cycles and lock value, so it can't be kSpinLockFree or
+  // kSpinLockHeld.  Make sure the value returned is at least
+  // kSpinLockSleeper.
+  wait_cycles |= kSpinLockSleeper;
+  return wait_cycles;
 }