Cap cpu idle exit latency for the life of the run, deep enough to keep C1 and shallow enough to block the rest

system.go

@@ -257,6 +257,7 @@ const (
 	wantCoalesceUsecs = 25
 	wantRxRing        = 8160
 	wantTxRing        = 4096
+	wantIdleLatency   = 10
 )
 
 type checkResult struct {
@@ -502,6 +503,31 @@ func checkGovernor(want string) checkResult {
 	return res
 }
 
+// Bounds how far into idle any cpu may sink, so a worker or interrupt never
+// waits behind a deep state's exit. Ten microseconds admits C1, whose exit is
+// noise under the interrupt coalescing already in the path, and blocks the
+// states whose exits are many times that window. The kernel honours the bound
+// for exactly as long as the fd stays open, which is why it is opened once and
+// deliberately never closed: as PID 1 its lifetime is the machine's, and
+// everything is restored the moment the process goes.
+func checkIdleLatency(usecs uint32) checkResult {
+	res := checkResult{item: "cpu idle latency"}
+	fd, err := unix.Open("/dev/cpu_dma_latency", unix.O_RDWR|unix.O_CLOEXEC, 0)
+	if err != nil {
+		res.err = fmt.Errorf("opening /dev/cpu_dma_latency: %w", err)
+		return res
+	}
+	var buf [4]byte
+	binary.NativeEndian.PutUint32(buf[:], usecs)
+	if _, err := unix.Write(fd, buf[:]); err != nil {
+		unix.Close(fd)
+		res.err = fmt.Errorf("writing cpu latency bound: %w", err)
+		return res
+	}
+	res.state = fmt.Sprintf("capped at %dus while running", usecs)
+	return res
+}
+
 func checkLinkUp(fd int, ifname string) checkResult {
 	res := checkResult{item: ifname + " link up"}
 	var ifr flagsIfreq
@@ -601,7 +627,7 @@ func withIoctlSocket(fn func(fd int) []checkResult) []checkResult {
 // under test, so with no cable there is never going to be one.
 func configureSystem(ifnames []string, ethertypes []uint16) []checkResult {
 	return withIoctlSocket(func(fd int) []checkResult {
-		out := []checkResult{checkGovernor(wantGovernor)}
+		out := []checkResult{checkGovernor(wantGovernor), checkIdleLatency(wantIdleLatency)}
 		for _, ifname := range ifnames {
 			out = append(out, checkLinkUp(fd, ifname))
 			out = append(out, clearFlowRules(fd, ifname))