rcutorture: Don't wait for kernel when all builds fail

Currently, rcutorture groups runs in batches, building each scenario in
a given batch, then invoking qemu to run all the kernels in the batch.
Of course, if a given scenario's kernel fails to build, there is no qemu
run for that scenario.  And if all of the kernels in a given batch fail
to build, there are no runs, and rcutorture immediately starts on the
next batch.

But not if --jitter has been specified, which it is by default.  In this
case, the jitter scripts are started unconditionally, and rcutorture
waits for them to complete, even though there are no kernels to run.

This commit therefore checks for this situation, and refuses to start
jitter unless at least one of the kernels in the batch built successfully.
This saves substantial time when all scenarios' kernels fail to build,
particularly if a long --duration was specified.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

tools/testing/selftests/rcutorture/bin/kvm-test-1-run.sh

@@ -96,6 +96,8 @@ then
 	KERNEL=$base_resdir/${BOOT_IMAGE##*/} # use the last component of ${BOOT_IMAGE}
 	ln -s $base_resdir/Make*.out $resdir  # for kvm-recheck.sh
 	ln -s $base_resdir/.config $resdir  # for kvm-recheck.sh
+	# Arch-independent indicator
+	touch $resdir/builtkernel
 elif kvm-build.sh $T/Kc2 $builddir
 then
 	# Had to build a kernel for this test.
@@ -108,6 +110,8 @@ then
 	then
 		cp $builddir/$BOOT_IMAGE $resdir
 		KERNEL=$resdir/${BOOT_IMAGE##*/}
+		# Arch-independent indicator
+		touch $resdir/builtkernel
 	else
 		echo No identifiable boot image, not running KVM, see $resdir.
 		echo Do the torture scripts know about your architecture?

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

@@ -332,6 +332,7 @@ function dump(first, pastlast, batchnum)
 {
 	print "echo ----Start batch " batchnum ": `date`";
 	print "echo ----Start batch " batchnum ": `date` >> " rd "/log";
+	print "needqemurun="
 	jn=1
 	for (j = first; j < pastlast; j++) {
 		builddir=KVM "/b" jn
@@ -367,10 +368,11 @@ function dump(first, pastlast, batchnum)
 	for (j = 1; j < jn; j++) {
 		builddir=KVM "/b" j
 		print "rm -f " builddir ".ready"
-		print "if test -z \"$TORTURE_BUILDONLY\""
+		print "if test -f \"" rd cfr[j] "/builtkernel\""
 		print "then"
-		print "\techo ----", cfr[j], cpusr[j] ovf ": Starting kernel. `date`";
-		print "\techo ----", cfr[j], cpusr[j] ovf ": Starting kernel. `date` >> " rd "/log";
+		print "\techo ----", cfr[j], cpusr[j] ovf ": Kernel present. `date`";
+		print "\techo ----", cfr[j], cpusr[j] ovf ": Kernel present. `date` >> " rd "/log";
+		print "\tneedqemurun=1"
 		print "fi"
 	}
 	njitter = 0;
@@ -385,13 +387,22 @@ function dump(first, pastlast, batchnum)
 		njitter = 0;
 		print "echo Build-only run, so suppressing jitter >> " rd "/log"
 	}
-	for (j = 0; j < njitter; j++)
-		print "jitter.sh " j " " dur " " ja[2] " " ja[3] "&"
-	print "wait"
-	print "if test -z \"$TORTURE_BUILDONLY\""
+	if (TORTURE_BUILDONLY) {
+		print "needqemurun="
+	}
+	print "if test -n \"$needqemurun\""
 	print "then"
+	print "\techo ---- Starting kernels. `date`";
+	print "\techo ---- Starting kernels. `date` >> " rd "/log";
+	for (j = 0; j < njitter; j++)
+		print "\tjitter.sh " j " " dur " " ja[2] " " ja[3] "&"
+	print "\twait"
 	print "\techo ---- All kernel runs complete. `date`";
 	print "\techo ---- All kernel runs complete. `date` >> " rd "/log";
+	print "else"
+	print "\twait"
+	print "\techo ---- No kernel runs. `date`";
+	print "\techo ---- No kernel runs. `date` >> " rd "/log";
 	print "fi"
 	for (j = 1; j < jn; j++) {
 		builddir=KVM "/b" j