runtime: move pdesc into p

There are currently two arrays indexed by P ID: allp and pdesc.
Consolidate these by moving the pdesc fields into type p so they can
be indexed off allp along with all other per-P state.

For #15131.

Change-Id: Ib6c4e6e7612281a1171ba4a0d62e52fd59e960b4
Reviewed-on: https://go-review.googlesource.com/45572
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/proc.go

@@ -3837,7 +3837,7 @@ func sysmon() {
 	}
 }
 
-var pdesc [_MaxGomaxprocs]struct {
+type sysmontick struct {
 	schedtick   uint32
 	schedwhen   int64
 	syscalltick uint32
@@ -3855,7 +3855,7 @@ func retake(now int64) uint32 {
 		if _p_ == nil {
 			continue
 		}
-		pd := &pdesc[i]
+		pd := &_p_.sysmontick
 		s := _p_.status
 		if s == _Psyscall {
 			// Retake P from syscall if it's there for more than 1 sysmon tick (at least 20us).

src/runtime/runtime2.go

@@ -453,9 +453,10 @@ type p struct {
 	id          int32
 	status      uint32 // one of pidle/prunning/...
 	link        puintptr
-	schedtick   uint32   // incremented on every scheduler call
-	syscalltick uint32   // incremented on every system call
-	m           muintptr // back-link to associated m (nil if idle)
+	schedtick   uint32     // incremented on every scheduler call
+	syscalltick uint32     // incremented on every system call
+	sysmontick  sysmontick // last tick observed by sysmon
+	m           muintptr   // back-link to associated m (nil if idle)
 	mcache      *mcache
 	racectx     uintptr