runtime: fix gcpacertrace printing of sweep ratio

Commit 44ed88a5 moved printing of the "sweep done" gcpacertrace
message so that it is printed when the final sweeper finishes.
However, by this point some other thread has often already observed
that there are no more spans to sweep and zeroed sweepPagesPerByte.

Avoid printing a 0 sweep ratio in the trace when this race happens by
getting the value of the sweep ratio upon entry to sweepone and
printing that.

Change-Id: Iac0c48ae899e12f193267cdfb012c921f8b71c85
Reviewed-on: https://go-review.googlesource.com/39492
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgcsweep.go

@@ -74,6 +74,7 @@ func bgsweep(c chan int) {
 //go:nowritebarrier
 func sweepone() uintptr {
 	_g_ := getg()
+	sweepRatio := mheap_.sweepPagesPerByte // For debugging
 
 	// increment locks to ensure that the goroutine is not preempted
 	// in the middle of sweep thus leaving the span in an inconsistent state for next GC
@@ -119,7 +120,7 @@ func sweepone() uintptr {
 	// last one print trace information.
 	if atomic.Xadd(&mheap_.sweepers, -1) == 0 && atomic.Load(&mheap_.sweepdone) != 0 {
 		if debug.gcpacertrace > 0 {
-			print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", mheap_.spanBytesAlloc>>20, "MB of spans; swept ", mheap_.pagesSwept, " pages at ", mheap_.sweepPagesPerByte, " pages/byte\n")
+			print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", mheap_.spanBytesAlloc>>20, "MB of spans; swept ", mheap_.pagesSwept, " pages at ", sweepRatio, " pages/byte\n")
 		}
 	}
 	_g_.m.locks--