Set fastPathContextLimit after maxSysmsgThreads has been initialized.

A recent change to the timing of initializing maxSysmsgThreads did not
account for fastPathContextLimit which depended on it.

Also set these globals only if they haven't been set already. We don't
expect there to be more than one instance of systrap, but just in case.

PiperOrigin-RevId: 633783350

pkg/sentry/platform/systrap/metrics.go

@@ -257,12 +257,16 @@ var (
 	// deepSleepTimeout is much shorter than the Linux scheduler timeslice, so the
 	// only thing that matters here is whether the Sentry handles syscall faster
 	// than the overhead of scheduling another stub thread.
-	fastPathContextLimit = uint32(maxSysmsgThreads * 2)
+	//
+	// It is set after maxSysmsgThreads is initialized.
+	fastPathContextLimit = uint32(0)
 )
 
 // controlFastPath is used to spawn a goroutine when creating the Systrap
 // platform.
 func controlFastPath() {
+	fastPathContextLimit = uint32(maxSysmsgThreads * 2)
+
 	for {
 		time.Sleep(recordingPeriod)
 

pkg/sentry/platform/systrap/systrap.go

@@ -316,12 +316,14 @@ func (*Systrap) MinUserAddress() hostarch.Addr {
 
 // New returns a new seccomp-based implementation of the platform interface.
 func New() (*Systrap, error) {
-	// CPUID information has been initialized at this point.
-	archState.Init()
-	// GOMAXPROCS has been set at this point.
-	maxSysmsgThreads = runtime.GOMAXPROCS(0)
-	// Account for syscall thread.
-	maxChildThreads = maxSysmsgThreads + 1
+	if maxSysmsgThreads == 0 {
+		// CPUID information has been initialized at this point.
+		archState.Init()
+		// GOMAXPROCS has been set at this point.
+		maxSysmsgThreads = runtime.GOMAXPROCS(0)
+		// Account for syscall thread.
+		maxChildThreads = maxSysmsgThreads + 1
+	}
 
 	mf, err := createMemoryFile()
 	if err != nil {