rcutorture: Allow a negative value for nfakewriters

Currently "nfakewriters" parameter can be set to any value but
there is no possibility to adjust it automatically based on how
many CPUs a system has where a test is run on.

To address this, if the "nfakewriters" is set to negative it will
be adjusted to num_online_cpus() during torture initialization.

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Link: https://lore.kernel.org/r/20250227131613.52683-1-urezki@gmail.com
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Author: urezki

kernel/rcu/rcutorture.c

@@ -147,6 +147,7 @@ MODULE_PARM_DESC(torture_type, "Type of RCU to torture (rcu, srcu, ...)");
 
 static int nrealnocbers;
 static int nrealreaders;
+static int nrealfakewriters;
 static struct task_struct *writer_task;
 static struct task_struct **fakewriter_tasks;
 static struct task_struct **reader_tasks;
@@ -1728,7 +1729,7 @@ rcu_torture_fakewriter(void *arg)
 	do {
 		torture_hrtimeout_jiffies(torture_random(&rand) % 10, &rand);
 		if (cur_ops->cb_barrier != NULL &&
-		    torture_random(&rand) % (nfakewriters * 8) == 0) {
+		    torture_random(&rand) % (nrealfakewriters * 8) == 0) {
 			cur_ops->cb_barrier();
 		} else {
 			switch (synctype[torture_random(&rand) % nsynctypes]) {
@@ -2522,7 +2523,7 @@ rcu_torture_print_module_parms(struct rcu_torture_ops *cur_ops, const char *tag)
 		 "nocbs_nthreads=%d nocbs_toggle=%d "
 		 "test_nmis=%d "
 		 "preempt_duration=%d preempt_interval=%d\n",
-		 torture_type, tag, nrealreaders, nfakewriters,
+		 torture_type, tag, nrealreaders, nrealfakewriters,
 		 stat_interval, verbose, test_no_idle_hz, shuffle_interval,
 		 stutter, irqreader, fqs_duration, fqs_holdoff, fqs_stutter,
 		 test_boost, cur_ops->can_boost,
@@ -3597,7 +3598,7 @@ rcu_torture_cleanup(void)
 	rcu_torture_reader_mbchk = NULL;
 
 	if (fakewriter_tasks) {
-		for (i = 0; i < nfakewriters; i++)
+		for (i = 0; i < nrealfakewriters; i++)
 			torture_stop_kthread(rcu_torture_fakewriter,
 					     fakewriter_tasks[i]);
 		kfree(fakewriter_tasks);
@@ -3994,6 +3995,14 @@ rcu_torture_init(void)
 
 	rcu_torture_init_srcu_lockdep();
 
+	if (nfakewriters >= 0) {
+		nrealfakewriters = nfakewriters;
+	} else {
+		nrealfakewriters = num_online_cpus() - 2 - nfakewriters;
+		if (nrealfakewriters <= 0)
+			nrealfakewriters = 1;
+	}
+
 	if (nreaders >= 0) {
 		nrealreaders = nreaders;
 	} else {
@@ -4050,8 +4059,9 @@ rcu_torture_init(void)
 					  writer_task);
 	if (torture_init_error(firsterr))
 		goto unwind;
-	if (nfakewriters > 0) {
-		fakewriter_tasks = kcalloc(nfakewriters,
+
+	if (nrealfakewriters > 0) {
+		fakewriter_tasks = kcalloc(nrealfakewriters,
 					   sizeof(fakewriter_tasks[0]),
 					   GFP_KERNEL);
 		if (fakewriter_tasks == NULL) {
@@ -4060,7 +4070,7 @@ rcu_torture_init(void)
 			goto unwind;
 		}
 	}
-	for (i = 0; i < nfakewriters; i++) {
+	for (i = 0; i < nrealfakewriters; i++) {
 		firsterr = torture_create_kthread(rcu_torture_fakewriter,
 						  NULL, fakewriter_tasks[i]);
 		if (torture_init_error(firsterr))