runtime: eliminate contention during stack allocation

Standard-sized stack frames use plain malloc/free
instead of centralized lock-protected FixAlloc.
Benchmark results on HP Z600 (2 x Xeon E5620, 8 HT cores, 2.40GHz)
are as follows:
benchmark                                        old ns/op    new ns/op    delta
BenchmarkStackGrowth                               1045.00       949.00   -9.19%
BenchmarkStackGrowth-2                             3450.00       800.00  -76.81%
BenchmarkStackGrowth-4                             5076.00       513.00  -89.89%
BenchmarkStackGrowth-8                             7805.00       471.00  -93.97%
BenchmarkStackGrowth-16                           11751.00       321.00  -97.27%

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4657091

src/pkg/runtime/malloc.goc

@@ -358,26 +358,11 @@ func new(n uint32) (ret *uint8) {
 	ret = runtime·mal(n);
 }
 
-// Stack allocator uses malloc/free most of the time,
-// but if we're in the middle of malloc and need stack,
-// we have to do something else to avoid deadlock.
-// In that case, we fall back on a fixed-size free-list
-// allocator, assuming that inside malloc all the stack
-// frames are small, so that all the stack allocations
-// will be a single size, the minimum (right now, 5k).
-static struct {
-	Lock;
-	FixAlloc;
-} stacks;
-
-enum {
-	FixedStack = StackMin,
-};
-
 void*
 runtime·stackalloc(uint32 n)
 {
 	void *v;
+	uintptr sys0;
 
 	// Stackalloc must be called on scheduler stack, so that we
 	// never try to grow the stack during the code that stackalloc runs.
@@ -385,18 +370,22 @@ runtime·stackalloc(uint32 n)
 	if(g != m->g0)
 		runtime·throw("stackalloc not on scheduler stack");
 
+	// Stack allocator uses malloc/free most of the time,
+	// but if we're in the middle of malloc and need stack,
+	// we have to do something else to avoid deadlock.
+	// In that case, we fall back on a fixed-size free-list
+	// allocator, assuming that inside malloc all the stack
+	// frames are small, so that all the stack allocations
+	// will be a single size, the minimum (right now, 5k).
 	if(m->mallocing || m->gcing || n == FixedStack) {
-		runtime·lock(&stacks);
-		if(stacks.size == 0)
-			runtime·FixAlloc_Init(&stacks, n, runtime·SysAlloc, nil, nil);
-		if(stacks.size != n) {
-			runtime·printf("stackalloc: in malloc, size=%D want %d", (uint64)stacks.size, n);
+		if(n != FixedStack) {
+			runtime·printf("stackalloc: in malloc, size=%d want %d", FixedStack, n);
 			runtime·throw("stackalloc");
 		}
-		v = runtime·FixAlloc_Alloc(&stacks);
-		mstats.stacks_inuse = stacks.inuse;
-		mstats.stacks_sys = stacks.sys;
-		runtime·unlock(&stacks);
+		sys0 = m->stackalloc->sys;
+		v = runtime·FixAlloc_Alloc(m->stackalloc);
+		mstats.stacks_inuse += FixedStack;
+		mstats.stacks_sys += m->stackalloc->sys - sys0;
 		return v;
 	}
 	return runtime·mallocgc(n, FlagNoProfiling|FlagNoGC, 0, 0);
@@ -405,12 +394,13 @@ runtime·stackalloc(uint32 n)
 void
 runtime·stackfree(void *v, uintptr n)
 {
+	uintptr sys0;
+
 	if(m->mallocing || m->gcing || n == FixedStack) {
-		runtime·lock(&stacks);
-		runtime·FixAlloc_Free(&stacks, v);
-		mstats.stacks_inuse = stacks.inuse;
-		mstats.stacks_sys = stacks.sys;
-		runtime·unlock(&stacks);
+		sys0 = m->stackalloc->sys;
+		runtime·FixAlloc_Free(m->stackalloc, v);
+		mstats.stacks_inuse -= FixedStack;
+		mstats.stacks_sys += m->stackalloc->sys - sys0;
 		return;
 	}
 	runtime·free(v);

src/pkg/runtime/malloc.h

@@ -80,7 +80,6 @@
 // This C code was written with an eye toward translating to Go
 // in the future.  Methods have the form Type_Method(Type *t, ...).
 
-typedef struct FixAlloc	FixAlloc;
 typedef struct MCentral	MCentral;
 typedef struct MHeap	MHeap;
 typedef struct MSpan	MSpan;

src/pkg/runtime/proc.c

@@ -97,6 +97,7 @@ static G* gfget(void);
 static void matchmg(void);	// match ms to gs
 static void readylocked(G*);	// ready, but sched is locked
 static void mnextg(M*, G*);
+static void mcommoninit(M*);
 
 // The bootstrap sequence is:
 //
@@ -116,11 +117,10 @@ runtime·schedinit(void)
 	int32 n;
 	byte *p;
 
-	runtime·allm = m;
 	m->nomemprof++;
-	m->fastrand = 0x49f6428aUL + m->id;
-
 	runtime·mallocinit();
+	mcommoninit(m);
+
 	runtime·goargs();
 	runtime·goenvs();
 
@@ -134,7 +134,6 @@ runtime·schedinit(void)
 	if(p != nil && (n = runtime·atoi(p)) != 0)
 		runtime·gomaxprocs = n;
 	runtime·sched.mcpumax = runtime·gomaxprocs;
-	runtime·sched.mcount = 1;
 	runtime·sched.predawn = 1;
 
 	m->nomemprof--;
@@ -208,6 +207,17 @@ runtime·idlegoroutine(void)
 	g->idlem = m;
 }
 
+static void
+mcommoninit(M *m)
+{
+	m->alllink = runtime·allm;
+	runtime·allm = m;
+	m->id = runtime·sched.mcount++;
+	m->fastrand = 0x49f6428aUL + m->id;
+	m->stackalloc = runtime·malloc(sizeof(*m->stackalloc));
+	runtime·FixAlloc_Init(m->stackalloc, FixedStack, runtime·SysAlloc, nil, nil);
+}
+
 // Put on `g' queue.  Sched must be locked.
 static void
 gput(G *g)
@@ -494,10 +504,7 @@ matchmg(void)
 			m = runtime·malloc(sizeof(M));
 			// Add to runtime·allm so garbage collector doesn't free m
 			// when it is just in a register or thread-local storage.
-			m->alllink = runtime·allm;
-			runtime·allm = m;
-			m->id = runtime·sched.mcount++;
-			m->fastrand = 0x49f6428aUL + m->id;
+			mcommoninit(m);
 
 			if(runtime·iscgo) {
 				CgoThreadStart ts;

src/pkg/runtime/proc_test.go

@@ -6,6 +6,7 @@ package runtime_test
 
 import (
 	"runtime"
+	"sync/atomic"
 	"testing"
 )
 
@@ -44,3 +45,31 @@ func TestStopTheWorldDeadlock(t *testing.T) {
 	stop <- true
 	runtime.GOMAXPROCS(maxprocs)
 }
+
+func stackGrowthRecursive(i int) {
+	var pad [128]uint64
+	if i != 0 && pad[0] == 0 {
+		stackGrowthRecursive(i - 1)
+	}
+}
+
+func BenchmarkStackGrowth(b *testing.B) {
+	const CallsPerSched = 1000
+	procs := runtime.GOMAXPROCS(-1)
+	N := int32(b.N / CallsPerSched)
+	c := make(chan bool, procs)
+	for p := 0; p < procs; p++ {
+		go func() {
+			for atomic.AddInt32(&N, -1) >= 0 {
+				runtime.Gosched()
+				for g := 0; g < CallsPerSched; g++ {
+					stackGrowthRecursive(10)
+				}
+			}
+			c <- true
+		}()
+	}
+	for p := 0; p < procs; p++ {
+		<-c
+	}
+}

src/pkg/runtime/runtime.h

@@ -57,6 +57,7 @@ typedef	struct	String		String;
 typedef	struct	Usema		Usema;
 typedef	struct	SigTab		SigTab;
 typedef	struct	MCache		MCache;
+typedef struct	FixAlloc	FixAlloc;
 typedef	struct	Iface		Iface;
 typedef	struct	Itab		Itab;
 typedef	struct	Eface		Eface;
@@ -236,6 +237,7 @@ struct	M
 	M*	schedlink;
 	uint32	machport;	// Return address for Mach IPC (OS X)
 	MCache	*mcache;
+	FixAlloc	*stackalloc;
 	G*	lockedg;
 	G*	idleg;
 	uint32	freglo[16];	// D[i] lsb and F[i]

src/pkg/runtime/stack.h

@@ -71,6 +71,7 @@ enum {
 	// If the amount needed for the splitting frame + StackExtra
 	// is less than this number, the stack will have this size instead.
 	StackMin = 4096,
+	FixedStack = StackMin,
 
 	// Functions that need frames bigger than this call morestack
 	// unconditionally.  That is, on entry to a function it is assumed