runtime: do not collect GC roots explicitly

Currently we collect (add) all roots into a global array in a single-threaded GC phase.
This hinders parallelism.
With this change we just kick off parallel for for number_of_goroutines+5 iterations.
Then parallel for callback decides whether it needs to scan stack of a goroutine
scan data segment, scan finalizers, etc. This eliminates the single-threaded phase entirely.
This requires to store all goroutines in an array instead of a linked list
(to allow direct indexing).
This CL also removes DebugScan functionality. It is broken because it uses
unbounded stack, so it can not run on g0. When it was working, I've found
it helpless for debugging issues because the two algorithms are too different now.
This change would require updating the DebugScan, so it's simpler to just delete it.

With 8 threads this change reduces GC pause by ~6%, while keeping cputime roughly the same.

garbage-8
allocated                 2987886      2989221      +0.04%
allocs                      62885        62887      +0.00%
cputime                  21286000     21272000      -0.07%
gc-pause-one             26633247     24885421      -6.56%
gc-pause-total             873570       811264      -7.13%
rss                     242089984    242515968      +0.18%
sys-gc                   13934336     13869056      -0.47%
sys-heap                205062144    205062144      +0.00%
sys-other                12628288     12628288      +0.00%
sys-stack                11534336     11927552      +3.41%
sys-total               243159104    243487040      +0.13%
time                      2809477      2740795      -2.44%

R=golang-codereviews, rsc
CC=cshapiro, golang-codereviews, khr
https://golang.org/cl/46860043

src/pkg/runtime/mprof.goc

@@ -436,7 +436,7 @@ saveg(uintptr pc, uintptr sp, G *gp, TRecord *r)
 }
 
 func GoroutineProfile(b Slice) (n int, ok bool) {
-	uintptr pc, sp;
+	uintptr pc, sp, i;
 	TRecord *r;
 	G *gp;
 	
@@ -455,7 +455,8 @@ func GoroutineProfile(b Slice) (n int, ok bool) {
 			ok = true;
 			r = (TRecord*)b.array;
 			saveg(pc, sp, g, r++);
-			for(gp = runtime·allg; gp != nil; gp = gp->alllink) {
+			for(i = 0; i < runtime·allglen; i++) {
+				gp = runtime·allg[i];
 				if(gp == g || gp->status == Gdead)
 					continue;
 				saveg(~(uintptr)0, ~(uintptr)0, gp, r++);

src/pkg/runtime/proc.c

@@ -67,8 +67,7 @@ int32	runtime·gomaxprocs;
 uint32	runtime·needextram;
 bool	runtime·iscgo;
 M	runtime·m0;
-G	runtime·g0;	 // idle goroutine for m0
-G*	runtime·allg;
+G	runtime·g0;	// idle goroutine for m0
 G*	runtime·lastg;
 M*	runtime·allm;
 M*	runtime·extram;
@@ -76,6 +75,11 @@ int8*	runtime·goos;
 int32	runtime·ncpu;
 static int32	newprocs;
 
+static	Lock allglock;	// the following vars are protected by this lock or by stoptheworld
+G**	runtime·allg;
+uintptr runtime·allglen;
+static	uintptr allgcap;
+
 void runtime·mstart(void);
 static void runqput(P*, G*);
 static G* runqget(P*);
@@ -115,6 +119,7 @@ static bool preemptall(void);
 static bool preemptone(P*);
 static bool exitsyscallfast(void);
 static bool haveexperiment(int8*);
+static void allgadd(G*);
 
 // The bootstrap sequence is:
 //
@@ -278,6 +283,7 @@ runtime·tracebackothers(G *me)
 {
 	G *gp;
 	int32 traceback;
+	uintptr i;
 
 	traceback = runtime·gotraceback(nil);
 	
@@ -288,7 +294,9 @@ runtime·tracebackothers(G *me)
 		runtime·traceback(~(uintptr)0, ~(uintptr)0, 0, gp);
 	}
 
-	for(gp = runtime·allg; gp != nil; gp = gp->alllink) {
+	runtime·lock(&allglock);
+	for(i = 0; i < runtime·allglen; i++) {
+		gp = runtime·allg[i];
 		if(gp == me || gp == m->curg || gp->status == Gdead)
 			continue;
 		if(gp->issystem && traceback < 2)
@@ -301,6 +309,7 @@ runtime·tracebackothers(G *me)
 		} else
 			runtime·traceback(~(uintptr)0, ~(uintptr)0, 0, gp);
 	}
+	runtime·unlock(&allglock);
 }
 
 static void
@@ -792,13 +801,7 @@ runtime·newextram(void)
 	if(raceenabled)
 		gp->racectx = runtime·racegostart(runtime·newextram);
 	// put on allg for garbage collector
-	runtime·lock(&runtime·sched);
-	if(runtime·lastg == nil)
-		runtime·allg = gp;
-	else
-		runtime·lastg->alllink = gp;
-	runtime·lastg = gp;
-	runtime·unlock(&runtime·sched);
+	allgadd(gp);
 
 	// Add m to the extra list.
 	mnext = lockextra(true);
@@ -1766,13 +1769,7 @@ runtime·newproc1(FuncVal *fn, byte *argp, int32 narg, int32 nret, void *callerp
 			runtime·throw("invalid stack in newg");
 	} else {
 		newg = runtime·malg(StackMin);
-		runtime·lock(&runtime·sched);
-		if(runtime·lastg == nil)
-			runtime·allg = newg;
-		else
-			runtime·lastg->alllink = newg;
-		runtime·lastg = newg;
-		runtime·unlock(&runtime·sched);
+		allgadd(newg);
 	}
 
 	sp = (byte*)newg->stackbase;
@@ -1805,6 +1802,31 @@ runtime·newproc1(FuncVal *fn, byte *argp, int32 narg, int32 nret, void *callerp
 	return newg;
 }
 
+static void
+allgadd(G *gp)
+{
+	G **new;
+	uintptr cap;
+
+	runtime·lock(&allglock);
+	if(runtime·allglen >= allgcap) {
+		cap = 4096/sizeof(new[0]);
+		if(cap < 2*allgcap)
+			cap = 2*allgcap;
+		new = runtime·malloc(cap*sizeof(new[0]));
+		if(new == nil)
+			runtime·throw("runtime: cannot allocate memory");
+		if(runtime·allg != nil) {
+			runtime·memmove(new, runtime·allg, runtime·allglen*sizeof(new[0]));
+			runtime·free(runtime·allg);
+		}
+		runtime·allg = new;
+		allgcap = cap;
+	}
+	runtime·allg[runtime·allglen++] = gp;
+	runtime·unlock(&allglock);
+}
+
 // Put on gfree list.
 // If local list is too long, transfer a batch to the global list.
 static void
@@ -1994,19 +2016,21 @@ runtime·gcount(void)
 {
 	G *gp;
 	int32 n, s;
+	uintptr i;
 
 	n = 0;
-	runtime·lock(&runtime·sched);
+	runtime·lock(&allglock);
 	// TODO(dvyukov): runtime.NumGoroutine() is O(N).
 	// We do not want to increment/decrement centralized counter in newproc/goexit,
 	// just to make runtime.NumGoroutine() faster.
 	// Compromise solution is to introduce per-P counters of active goroutines.
-	for(gp = runtime·allg; gp; gp = gp->alllink) {
+	for(i = 0; i < runtime·allglen; i++) {
+		gp = runtime·allg[i];
 		s = gp->status;
 		if(s == Grunnable || s == Grunning || s == Gsyscall || s == Gwaiting)
 			n++;
 	}
-	runtime·unlock(&runtime·sched);
+	runtime·unlock(&allglock);
 	return n;
 }
 
@@ -2345,6 +2369,7 @@ checkdead(void)
 {
 	G *gp;
 	int32 run, grunning, s;
+	uintptr i;
 
 	// -1 for sysmon
 	run = runtime·sched.mcount - runtime·sched.nmidle - runtime·sched.nmidlelocked - 1;
@@ -2356,17 +2381,21 @@ checkdead(void)
 		runtime·throw("checkdead: inconsistent counts");
 	}
 	grunning = 0;
-	for(gp = runtime·allg; gp; gp = gp->alllink) {
+	runtime·lock(&allglock);
+	for(i = 0; i < runtime·allglen; i++) {
+		gp = runtime·allg[i];
 		if(gp->isbackground)
 			continue;
 		s = gp->status;
 		if(s == Gwaiting)
 			grunning++;
 		else if(s == Grunnable || s == Grunning || s == Gsyscall) {
+			runtime·unlock(&allglock);
 			runtime·printf("checkdead: find g %D in status %d\n", gp->goid, s);
 			runtime·throw("checkdead: runnable g");
 		}
 	}
+	runtime·unlock(&allglock);
 	if(grunning == 0)  // possible if main goroutine calls runtime·Goexit()
 		runtime·exit(0);
 	m->throwing = -1;  // do not dump full stacks
@@ -2553,6 +2582,7 @@ runtime·schedtrace(bool detailed)
 	int64 now;
 	int64 id1, id2, id3;
 	int32 i, t, h;
+	uintptr gi;
 	int8 *fmt;
 	M *mp, *lockedm;
 	G *gp, *lockedg;
@@ -2620,13 +2650,16 @@ runtime·schedtrace(bool detailed)
 			mp->mallocing, mp->throwing, mp->gcing, mp->locks, mp->dying, mp->helpgc,
 			mp->spinning, id3);
 	}
-	for(gp = runtime·allg; gp; gp = gp->alllink) {
+	runtime·lock(&allglock);
+	for(gi = 0; gi < runtime·allglen; gi++) {
+		gp = runtime·allg[gi];
 		mp = gp->m;
 		lockedm = gp->lockedm;
 		runtime·printf("  G%D: status=%d(%s) m=%d lockedm=%d\n",
 			gp->goid, gp->status, gp->waitreason, mp ? mp->id : -1,
 			lockedm ? lockedm->id : -1);
 	}
+	runtime·unlock(&allglock);
 	runtime·unlock(&runtime·sched);
 }
 

src/pkg/runtime/runtime.h

@@ -263,7 +263,6 @@ struct	G
 	uintptr	stackguard;	// same as stackguard0, but not set to StackPreempt
 	uintptr	stack0;
 	uintptr	stacksize;
-	G*	alllink;	// on allg
 	void*	param;		// passed parameter on wakeup
 	int16	status;
 	int64	goid;
@@ -719,7 +718,8 @@ bool	runtime·topofstack(Func*);
  */
 extern	String	runtime·emptystring;
 extern	uintptr runtime·zerobase;
-extern	G*	runtime·allg;
+extern	G**	runtime·allg;
+extern	uintptr runtime·allglen;
 extern	G*	runtime·lastg;
 extern	M*	runtime·allm;
 extern	P**	runtime·allp;