runtime: don't call lockOSThread for every syscall call on Windows

Windows syscall.SyscallN currently calls lockOSThread for every syscall.
This can be expensive and produce unnecessary context switches,
especially when the syscall is called frequently under high contention.

The lockOSThread was necessary to ensure that cgocall wouldn't
reschedule the goroutine to a different M, as the syscall return values
are reported back in the M struct.

This CL instructs cgocall to copy the syscall return values into the
the M that will see the caller on return, so the caller no longer needs
to call lockOSThread.

Updates #58336.

Cq-Include-Trybots: luci.golang.try:gotip-windows-arm64,gotip-windows-amd64-longtest
Change-Id: If6644fd111dbacab74e7dcee2afa18ca146735da
Reviewed-on: https://go-review.googlesource.com/c/go/+/562915
Reviewed-by: Alex Brainman <[email protected]>
Auto-Submit: Emmanuel Odeke <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Run-TryBot: Emmanuel Odeke <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: qmuntal

src/runtime/cgocall.go

@@ -181,8 +181,14 @@ func cgocall(fn, arg unsafe.Pointer) int32 {
 
 	osPreemptExtExit(mp)
 
+	// Save current syscall parameters, so m.winsyscall can be
+	// used again if callback decide to make syscall.
+	winsyscall := mp.winsyscall
+
 	exitsyscall()
 
+	getg().m.winsyscall = winsyscall
+
 	// Note that raceacquire must be called only after exitsyscall has
 	// wired this M to a P.
 	if raceenabled {
@@ -297,9 +303,9 @@ func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) {
 
 	checkm := gp.m
 
-	// Save current syscall parameters, so m.syscall can be
+	// Save current syscall parameters, so m.winsyscall can be
 	// used again if callback decide to make syscall.
-	syscall := gp.m.syscall
+	winsyscall := gp.m.winsyscall
 
 	// entersyscall saves the caller's SP to allow the GC to trace the Go
 	// stack. However, since we're returning to an earlier stack frame and
@@ -340,7 +346,7 @@ func cgocallbackg(fn, frame unsafe.Pointer, ctxt uintptr) {
 	// going back to cgo call
 	reentersyscall(savedpc, uintptr(savedsp))
 
-	gp.m.syscall = syscall
+	gp.m.winsyscall = winsyscall
 }
 
 func cgocallbackg1(fn, frame unsafe.Pointer, ctxt uintptr) {

src/runtime/nonwindows_stub.go

@@ -21,3 +21,8 @@ func osRelax(relax bool) {}
 // enableWER is called by setTraceback("wer").
 // Windows Error Reporting (WER) is only supported on Windows.
 func enableWER() {}
+
+// winlibcall is not implemented on non-Windows systems,
+// but it is used in non-OS-specific parts of the runtime.
+// Define it as an empty struct to avoid wasting stack space.
+type winlibcall struct{}

src/runtime/os_windows.go

@@ -214,6 +214,8 @@ func asmstdcall(fn unsafe.Pointer)
 
 var asmstdcallAddr unsafe.Pointer
 
+type winlibcall libcall
+
 func windowsFindfunc(lib uintptr, name []byte) stdFunction {
 	if name[len(name)-1] != 0 {
 		throw("usage")

src/runtime/runtime2.go

@@ -612,11 +612,11 @@ type m struct {
 
 	// these are here because they are too large to be on the stack
 	// of low-level NOSPLIT functions.
-	libcall   libcall
-	libcallpc uintptr // for cpu profiler
-	libcallsp uintptr
-	libcallg  guintptr
-	syscall   libcall // stores syscall parameters on windows
+	libcall    libcall
+	libcallpc  uintptr // for cpu profiler
+	libcallsp  uintptr
+	libcallg   guintptr
+	winsyscall winlibcall // stores syscall parameters on windows
 
 	vdsoSP uintptr // SP for traceback while in VDSO call (0 if not in call)
 	vdsoPC uintptr // PC for traceback while in VDSO call

src/runtime/syscall_windows.go

@@ -500,16 +500,18 @@ func syscall_SyscallN(fn uintptr, args ...uintptr) (r1, r2, err uintptr) {
 	}
 
 	// The cgocall parameters are stored in m instead of in
-	// the stack because the stack can move during if fn
+	// the stack because the stack can move during fn if it
 	// calls back into Go.
-	lockOSThread()
-	defer unlockOSThread()
-	c := &getg().m.syscall
+	c := &getg().m.winsyscall
 	c.fn = fn
 	c.n = uintptr(len(args))
 	if c.n != 0 {
 		c.args = uintptr(noescape(unsafe.Pointer(&args[0])))
 	}
 	cgocall(asmstdcallAddr, unsafe.Pointer(c))
+	// cgocall may reschedule us on to a different M,
+	// but it copies the return values into the new M's
+	// so we can read them from there.
+	c = &getg().m.winsyscall
 	return c.r1, c.r2, c.err
 }