torture: Make kvm-remote.sh give up on unresponsive system

Currently, a system that stops responding at the wrong time will hang
kvm-remote.sh.  This can happen when the system in question is forced
offline for maintenance, and there is currently no way for the user
to kick this script into moving ahead.  This commit therefore causes
kvm-remote.sh to wait at most 15 minutes for a non-responsive system,
that is, a system for which ssh gives an exit code of 255.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

tools/testing/selftests/rcutorture/bin/kvm-remote.sh

@@ -181,10 +181,11 @@ done
 
 # Function to check for presence of a file on the specified system.
 # Complain if the system cannot be reached, and retry after a wait.
-# Currently just waits forever if a machine disappears.
+# Currently just waits 15 minutes if a machine disappears.
 #
 # Usage: checkremotefile system pathname
 checkremotefile () {
+	local nsshfails=0
 	local ret
 	local sleeptime=60
 
@@ -195,6 +196,11 @@ checkremotefile () {
 		if test "$ret" -eq 255
 		then
 			echo " ---" ssh failure to $1 checking for file $2, retry after $sleeptime seconds. `date` | tee -a "$oldrun/remote-log"
+			nsshfails=$((nsshfails+1))
+			if ((nsshfails > 15))
+			then
+				return 255
+			fi
 		elif test "$ret" -eq 0
 		then
 			return 0
@@ -268,12 +274,23 @@ echo All batches started. `date` | tee -a "$oldrun/remote-log"
 for i in $systems
 do
 	echo " ---" Waiting for $i `date` | tee -a "$oldrun/remote-log"
-	while checkremotefile "$i" "$resdir/$ds/remote.run"
+	while :
 	do
+		checkremotefile "$i" "$resdir/$ds/remote.run"
+		ret=$?
+		if test "$ret" -eq 1
+		then
+			echo " ---" Collecting results from $i `date` | tee -a "$oldrun/remote-log"
+			( cd "$oldrun"; ssh -o BatchMode=yes $i "cd $rundir; tar -czf - kvm-remote-*.sh.out */console.log */kvm-test-1-run*.sh.out */qemu[_-]pid */qemu-retval */qemu-affinity; rm -rf $T > /dev/null 2>&1" | tar -xzf - )
+			break;
+		fi
+		if test "$ret" -eq 255
+		then
+			echo System $i persistent ssh failure, lost results `date` | tee -a "$oldrun/remote-log"
+			break;
+		fi
 		sleep 30
 	done
-	echo " ---" Collecting results from $i `date` | tee -a "$oldrun/remote-log"
-	( cd "$oldrun"; ssh -o BatchMode=yes $i "cd $rundir; tar -czf - kvm-remote-*.sh.out */console.log */kvm-test-1-run*.sh.out */qemu[_-]pid */qemu-retval */qemu-affinity; rm -rf $T > /dev/null 2>&1" | tar -xzf - )
 done
 
 ( kvm-end-run-stats.sh "$oldrun" "$starttime"; echo $? > $T/exitcode ) | tee -a "$oldrun/remote-log"