rcutorture: Avoid printing cpu=-1 for no-fault RCU boost failure

If a CPU runs throughout the stalled grace period without passing
through a quiescent state, RCU priority boosting cannot help.
The rcu_torture_boost_failed() function therefore prints a message
flagging the first such CPU.  However, if the stall was instead due to
(for example) RCU's grace-period kthread being starved of CPU, there will
be no such CPU, causing rcu_check_boost_fail() to instead pass back -1
through its cpup CPU-pointer parameter.

Therefore, the current message complains about a mythical CPU -1.

This commit therefore checks for this situation, and notes that all CPUs
have passed through a quiescent state.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Author: paulmckrcu

kernel/rcu/rcutorture.c

@@ -1059,8 +1059,13 @@ static bool rcu_torture_boost_failed(unsigned long gp_state, unsigned long *star
 			// At most one persisted message per boost test.
 			j = jiffies;
 			lp = READ_ONCE(last_persist);
-			if (time_after(j, lp + mininterval) && cmpxchg(&last_persist, lp, j) == lp)
-				pr_info("Boost inversion persisted: No QS from CPU %d\n", cpu);
+			if (time_after(j, lp + mininterval) &&
+			    cmpxchg(&last_persist, lp, j) == lp) {
+				if (cpu < 0)
+					pr_info("Boost inversion persisted: QS from all CPUs\n");
+				else
+					pr_info("Boost inversion persisted: No QS from CPU %d\n", cpu);
+			}
 			return false; // passed on a technicality
 		}
 		VERBOSE_TOROUT_STRING("rcu_torture_boost boosting failed");