runtime: remove rescan list

With the hybrid barrier, rescanning stacks is no longer necessary so
the rescan list is no longer necessary. Remove it.

This leaves the gcrescanstacks GODEBUG variable, since it's useful for
debugging, but changes it to simply walk all of the Gs to rescan
stacks rather than using the rescan list.

We could also remove g.gcscanvalid, which is effectively a distributed
rescan list. However, it's still useful for gcrescanstacks mode and it
adds little complexity, so we'll leave it in.

Fixes #17099.
Updates #17503.

Change-Id: I776d43f0729567335ef1bfd145b75c74de2cc7a9
Reviewed-on: https://go-review.googlesource.com/36619
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

1 files changed, 5 insertions, 29 deletions

diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index e71ebcd7a7..11e1e21291 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -432,16 +432,6 @@ func allgadd(gp *g) {
 	lock(&allglock)
 	allgs = append(allgs, gp)
 	allglen = uintptr(len(allgs))
-
-	// Grow GC rescan list if necessary.
-	if len(allgs) > cap(work.rescan.list) {
-		lock(&work.rescan.lock)
-		l := work.rescan.list
-		// Let append do the heavy lifting, but keep the
-		// length the same.
-		work.rescan.list = append(l[:cap(l)], 0)[:len(l)]
-		unlock(&work.rescan.lock)
-	}
 	unlock(&allglock)
 }
 
@@ -795,9 +785,8 @@ func casgstatus(gp *g, oldval, newval uint32) {
 			nextYield = nanotime() + yieldDelay/2
 		}
 	}
-	if newval == _Grunning && gp.gcscanvalid {
-		// Run queueRescan on the system stack so it has more space.
-		systemstack(func() { queueRescan(gp) })
+	if newval == _Grunning {
+		gp.gcscanvalid = false
 	}
 }
 
@@ -1481,9 +1470,8 @@ func oneNewExtraM() {
 	gp.syscallpc = gp.sched.pc
 	gp.syscallsp = gp.sched.sp
 	gp.stktopsp = gp.sched.sp
-	gp.gcscanvalid = true // fresh G, so no dequeueRescan necessary
+	gp.gcscanvalid = true
 	gp.gcscandone = true
-	gp.gcRescan = -1
 	// malg returns status as Gidle, change to Gsyscall before adding to allg
 	// where GC will see it.
 	casgstatus(gp, _Gidle, _Gsyscall)
@@ -2346,8 +2334,7 @@ func goexit0(gp *g) {
 	gp.labels = nil
 
 	// Note that gp's stack scan is now "valid" because it has no
-	// stack. We could dequeueRescan, but that takes a lock and
-	// isn't really necessary.
+	// stack.
 	gp.gcscanvalid = true
 	dropg()
 
@@ -2875,7 +2862,6 @@ func newproc1(fn *funcval, argp *uint8, narg int32, nret int32, callerpc uintptr
 	if newg == nil {
 		newg = malg(_StackMin)
 		casgstatus(newg, _Gidle, _Gdead)
-		newg.gcRescan = -1
 		allgadd(newg) // publishes with a g->status of Gdead so GC scanner doesn't look at uninitialized stack.
 	}
 	if newg.stack.hi == 0 {
@@ -2927,17 +2913,7 @@ func newproc1(fn *funcval, argp *uint8, narg int32, nret int32, callerpc uintptr
 	if isSystemGoroutine(newg) {
 		atomic.Xadd(&sched.ngsys, +1)
 	}
-	// The stack is dirty from the argument frame, so queue it for
-	// scanning. Do this before setting it to runnable so we still
-	// own the G. If we're recycling a G, it may already be on the
-	// rescan list.
-	if newg.gcRescan == -1 {
-		queueRescan(newg)
-	} else {
-		// The recycled G is already on the rescan list. Just
-		// mark the stack dirty.
-		newg.gcscanvalid = false
-	}
+	newg.gcscanvalid = false
 	casgstatus(newg, _Gdead, _Grunnable)
 
 	if _p_.goidcache == _p_.goidcacheend {