rcutorture: Forgive RCU boost failures when CPUs don't pass through QS

Currently, rcu_torture_boost() runs CPU-bound at real-time priority
to force RCU priority inversions.  It then checks that grace periods
progress during this CPU-bound time.  If grace periods fail to progress,
it reports and RCU priority boosting failure.

However, it is possible (and sometimes does happen) that the grace period
fails to progress due to a CPU failing to pass through a quiescent state
for an extended time period (3.5 seconds by default).  This can happen
due to vCPU preemption, long-running interrupts, and much else besides.
There is nothing that RCU priority boosting can do about these situations,
and so they should not be counted as RCU priority boosting failures.

This commit therefore checks for CPUs (as opposed to preempted tasks)
holding up a grace period, and flags the resulting RCU priority boosting
failures, but does not splat nor count them as errors.  It does rate-limit
them to avoid flooding the console log.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 41 insertions, 26 deletions

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 02a14dfcae67..5ae4dcc6ba27 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -340,6 +340,7 @@ struct rcu_torture_ops {
 	void (*fqs)(void);
 	void (*stats)(void);
 	void (*gp_kthread_dbg)(void);
+	bool (*check_boost_failed)(unsigned long gp_state, int *cpup);
 	int (*stall_dur)(void);
 	int irq_capable;
 	int can_boost;
@@ -483,31 +484,32 @@ static void rcu_sync_torture_init(void)
 }
 
 static struct rcu_torture_ops rcu_ops = {
-	.ttype		= RCU_FLAVOR,
-	.init		= rcu_sync_torture_init,
-	.readlock	= rcu_torture_read_lock,
-	.read_delay	= rcu_read_delay,
-	.readunlock	= rcu_torture_read_unlock,
-	.readlock_held	= torture_readlock_not_held,
-	.get_gp_seq	= rcu_get_gp_seq,
-	.gp_diff	= rcu_seq_diff,
-	.deferred_free	= rcu_torture_deferred_free,
-	.sync		= synchronize_rcu,
-	.exp_sync	= synchronize_rcu_expedited,
-	.get_gp_state	= get_state_synchronize_rcu,
-	.start_gp_poll	= start_poll_synchronize_rcu,
-	.poll_gp_state	= poll_state_synchronize_rcu,
-	.cond_sync	= cond_synchronize_rcu,
-	.call		= call_rcu,
-	.cb_barrier	= rcu_barrier,
-	.fqs		= rcu_force_quiescent_state,
-	.stats		= NULL,
-	.gp_kthread_dbg	= show_rcu_gp_kthreads,
-	.stall_dur	= rcu_jiffies_till_stall_check,
-	.irq_capable	= 1,
-	.can_boost	= IS_ENABLED(CONFIG_RCU_BOOST),
-	.extendables	= RCUTORTURE_MAX_EXTEND,
-	.name		= "rcu"
+	.ttype			= RCU_FLAVOR,
+	.init			= rcu_sync_torture_init,
+	.readlock		= rcu_torture_read_lock,
+	.read_delay		= rcu_read_delay,
+	.readunlock		= rcu_torture_read_unlock,
+	.readlock_held		= torture_readlock_not_held,
+	.get_gp_seq		= rcu_get_gp_seq,
+	.gp_diff		= rcu_seq_diff,
+	.deferred_free		= rcu_torture_deferred_free,
+	.sync			= synchronize_rcu,
+	.exp_sync		= synchronize_rcu_expedited,
+	.get_gp_state		= get_state_synchronize_rcu,
+	.start_gp_poll		= start_poll_synchronize_rcu,
+	.poll_gp_state		= poll_state_synchronize_rcu,
+	.cond_sync		= cond_synchronize_rcu,
+	.call			= call_rcu,
+	.cb_barrier		= rcu_barrier,
+	.fqs			= rcu_force_quiescent_state,
+	.stats			= NULL,
+	.gp_kthread_dbg		= show_rcu_gp_kthreads,
+	.check_boost_failed	= rcu_check_boost_fail,
+	.stall_dur		= rcu_jiffies_till_stall_check,
+	.irq_capable		= 1,
+	.can_boost		= IS_ENABLED(CONFIG_RCU_BOOST),
+	.extendables		= RCUTORTURE_MAX_EXTEND,
+	.name			= "rcu"
 };
 
 /*
@@ -918,14 +920,27 @@ static void rcu_torture_enable_rt_throttle(void)
 
 static bool rcu_torture_boost_failed(unsigned long gp_state, unsigned long start, unsigned long end)
 {
+	int cpu;
 	static int dbg_done;
 	bool gp_done;
+	unsigned long j;
+	static unsigned long last_persist;
+	unsigned long lp;
+	unsigned long mininterval = test_boost_duration * HZ - HZ / 2;
 
-	if (end - start > test_boost_duration * HZ - HZ / 2) {
+	if (end - start > mininterval) {
 		// Recheck after checking time to avoid false positives.
 		smp_mb(); // Time check before grace-period check.
 		if (cur_ops->poll_gp_state(gp_state))
 			return false; // passed, though perhaps just barely
+		if (cur_ops->check_boost_failed && !cur_ops->check_boost_failed(gp_state, &cpu)) {
+			// 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);
+			return false; // passed on a technicality
+		}
 		VERBOSE_TOROUT_STRING("rcu_torture_boost boosting failed");
 		n_rcu_torture_boost_failure++;
 		if (!xchg(&dbg_done, 1) && cur_ops->gp_kthread_dbg) {