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Bug 784859 - Part 2: Avoid locking to store the computed result in the global variable in CalibratedPerformanceCounter; r=bbondy

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This commit is contained in:
Ehsan Akhgari 2012-09-06 11:01:06 -04:00
parent cdd36264b3
commit c373a3df5a
1 changed files with 63 additions and 6 deletions
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69
xpcom/ds/TimeStamp_windows.cpp
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@ -29,12 +29,27 @@
static bool
HasStableTSC()
{
union {
int regs[4];
struct {
int nIds;
char cpuString[12];
};
} cpuInfo;
__cpuid(cpuInfo.regs, 0);
// Only allow Intel CPUs for now
// The order of the registers is reg[1], reg[3], reg[2]. We just adjust the
// string so that we can compare in one go.
if (_strnicmp(cpuInfo.cpuString, "GenuntelineI", sizeof(cpuInfo.cpuString)))
return false;
int regs[4];
// detect if the Advanced Power Management feature is supported
__cpuid(regs, 0x80000000);
if (regs[0] < 0x80000007)
return false;
return false;
__cpuid(regs, 0x80000007);
// if bit 8 is set than TSC will run at a constant rate
@ -201,6 +216,18 @@ static ULONGLONG
CalibratedPerformanceCounter();
static inline ULONGLONG
InterlockedRead64(volatile ULONGLONG* destination)
{
#ifdef _WIN64
// Aligned 64-bit reads on x86-64 are atomic
return *destination;
#else
// Dirty hack since Windows doesn't provide an atomic 64-bit read function
return _InterlockedCompareExchange64(reinterpret_cast<volatile __int64*> (destination), 0, 0);
#endif
}
// ----------------------------------------------------------------------------
// Critical Section helper class
// ----------------------------------------------------------------------------
@ -494,6 +521,36 @@ CheckCalibration(LONGLONG overflow, ULONGLONG qpc, ULONGLONG gtc)
return true;
}
// AtomicStoreIfGreaterThan tries to store the maximum of two values in one of them
// without locking. The only scenario in which two racing threads may corrupt the
// maximum value is when they both try to increase the value without knowing about
// each other, like below:
//
// Thread 1 reads 1000. newValue in thread 1 is 1005.
// Thread 2 reads 1000. newValue in thread 2 is 1001.
// Thread 1 tries to store. Its value is less than newValue, so the store happens.
// *destination is now 1005.
// Thread 2 tries to store. Its value is less than newValue, so the store happens.
// *destination is now 1001.
//
// The correct value to be stored if this was happening serially is 1005. The
// following algorithm achieves that.
//
// The return value is the maximum value.
ULONGLONG
AtomicStoreIfGreaterThan(ULONGLONG* destination, ULONGLONG newValue)
{
ULONGLONG readValue;
do {
readValue = InterlockedRead64(destination);
if (readValue > newValue)
return readValue;
} while (readValue != _InterlockedCompareExchange64(reinterpret_cast<volatile __int64*> (destination),
newValue, readValue));
return newValue;
}
// The main function. Result is in [mt] ensuring to not go back and be mostly
// reliable with highest possible resolution.
static ULONGLONG
@ -510,6 +567,8 @@ CalibratedPerformanceCounter()
ULONGLONG qpc = PerformanceCounter();
DWORD gtcw = GetTickCount();
ULONGLONG result;
{
AutoCriticalSection lock(&sTimeStampLock);
// Rollover protection
@ -525,7 +584,7 @@ CalibratedPerformanceCounter()
overflow = diff - sOverrunThreshold;
}
ULONGLONG result = qpc;
result = qpc;
if (!CheckCalibration(overflow, qpc, gtc)) {
// We are back on GTC, QPC has been observed unreliable
result = ms2mt(gtc) + sSkew;
@ -535,11 +594,9 @@ CalibratedPerformanceCounter()
LOG(("TimeStamp: result = %1.2fms, diff = %1.4fms",
mt2ms_d(result), mt2ms_d(diff)));
#endif
}
if (result > sLastResult)
sLastResult = result;
return sLastResult;
return AtomicStoreIfGreaterThan(&sLastResult, result);
}
// ----------------------------------------------------------------------------