scftorture: Pause testing after memory-allocation failure

The scftorture test can quickly execute a large number of calls to no-wait
smp_call_function(), each of which holds a block of memory until the
corresponding handler is invoked.  Especially when the longwait module
parameter is specified, this can chew up an arbitrarily large amount
of memory.  This commit therefore blocks after each memory-allocation
failure, with the duration a function of longwait.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/scftorture.c

@@ -313,6 +313,7 @@ static void scf_handler_1(void *scfc_in)
 // Randomly do an smp_call_function*() invocation.
 static void scftorture_invoke_one(struct scf_statistics *scfp, struct torture_random_state *trsp)
 {
+	bool allocfail = false;
 	uintptr_t cpu;
 	int ret = 0;
 	struct scf_check *scfcp = NULL;
@@ -327,6 +328,7 @@ static void scftorture_invoke_one(struct scf_statistics *scfp, struct torture_ra
 		if (!scfcp) {
 			WARN_ON_ONCE(!IS_ENABLED(CONFIG_KASAN));
 			atomic_inc(&n_alloc_errs);
+			allocfail = true;
 		} else {
 			scfcp->scfc_cpu = -1;
 			scfcp->scfc_wait = scfsp->scfs_wait;
@@ -433,7 +435,9 @@ static void scftorture_invoke_one(struct scf_statistics *scfp, struct torture_ra
 		cpus_read_unlock();
 	else
 		preempt_enable();
-	if (!(torture_random(trsp) & 0xfff))
+	if (allocfail)
+		schedule_timeout_idle((1 + longwait) * HZ);  // Let no-wait handlers complete.
+	else if (!(torture_random(trsp) & 0xfff))
 		schedule_timeout_uninterruptible(1);
 }