Message-ID: <20250329230143.62292-1-joelagnelf@nvidia.com>
Date: Sat, 29 Mar 2025 19:01:42 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	Davidlohr Bueso <dave@...olabs.net>
Cc: rcu@...r.kernel.org
Subject: [PATCH] rcutorture: Perform more frequent testing of ->gpwrap

Currently, the ->gpwrap is not tested (at all per my testing) due to the
requirement of a large delta between a CPU's rdp->gp_seq and its node's
rnp->gpseq.

This results in no testing of ->gpwrap being set. This patch by default
adds 5 minutes of testing with ->gpwrap forced by lowering the delta
between rdp->gp_seq and rnp->gp_seq to just 8 GPs. All of this is
configurable, including the active time for the setting and a full
testing cycle.

By default, the first 25 minutes of a test will have the _default_
behavior there is right now (ULONG_MAX / 4) delta. Then for 5 minutes,
we switch to a smaller delta causing 1-2 wraps in 5 minutes. I believe
this is reasonable since we at least add a little bit of testing for
usecases where ->gpwrap is set.

Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
---
 kernel/rcu/rcu.h        |  4 +++
 kernel/rcu/rcutorture.c | 64 +++++++++++++++++++++++++++++++++++++++++
 kernel/rcu/tree.c       | 34 ++++++++++++++++++++--
 kernel/rcu/tree.h       |  1 +
 4 files changed, 101 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index eed2951a4962..9a15e9701e02 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -572,6 +572,8 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
 			       unsigned long c_old,
 			       unsigned long c);
 void rcu_gp_set_torture_wait(int duration);
+void rcu_set_torture_ovf_lag(unsigned long lag);
+int rcu_get_gpwrap_count(int cpu);
 #else
 static inline void rcutorture_get_gp_data(int *flags, unsigned long *gp_seq)
 {
@@ -589,6 +591,8 @@ void do_trace_rcu_torture_read(const char *rcutorturename,
 	do { } while (0)
 #endif
 static inline void rcu_gp_set_torture_wait(int duration) { }
+static inline void rcu_set_torture_ovf_lag(unsigned long lag) { }
+static inline int rcu_get_gpwrap_count(int cpu) { return 0; }
 #endif
 unsigned long long rcutorture_gather_gp_seqs(void);
 void rcutorture_format_gp_seqs(unsigned long long seqs, char *cp, size_t len);
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 895a27545ae1..79a72e70913e 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -118,6 +118,9 @@ torture_param(int, nreaders, -1, "Number of RCU reader threads");
 torture_param(int, object_debug, 0, "Enable debug-object double call_rcu() testing");
 torture_param(int, onoff_holdoff, 0, "Time after boot before CPU hotplugs (s)");
 torture_param(int, onoff_interval, 0, "Time between CPU hotplugs (jiffies), 0=disable");
+torture_param(int, ovf_cycle_mins, 30, "Total cycle duration for ovf lag testing (in minutes)");
+torture_param(int, ovf_active_mins, 5, "Duration for which ovf lag is active within each cycle (in minutes)");
+torture_param(int, ovf_lag_gps, 8, "Value to set for set_torture_ovf_lag during an active testing period.");
 torture_param(int, nocbs_nthreads, 0, "Number of NOCB toggle threads, 0 to disable");
 torture_param(int, nocbs_toggle, 1000, "Time between toggling nocb state (ms)");
 torture_param(int, preempt_duration, 0, "Preemption duration (ms), zero to disable");
@@ -2629,6 +2632,7 @@ rcu_torture_stats_print(void)
 	int i;
 	long pipesummary[RCU_TORTURE_PIPE_LEN + 1] = { 0 };
 	long batchsummary[RCU_TORTURE_PIPE_LEN + 1] = { 0 };
+	long n_gpwraps = 0;
 	struct rcu_torture *rtcp;
 	static unsigned long rtcv_snap = ULONG_MAX;
 	static bool splatted;
@@ -2639,6 +2643,7 @@ rcu_torture_stats_print(void)
 			pipesummary[i] += READ_ONCE(per_cpu(rcu_torture_count, cpu)[i]);
 			batchsummary[i] += READ_ONCE(per_cpu(rcu_torture_batch, cpu)[i]);
 		}
+		n_gpwraps += rcu_get_gpwrap_count(cpu);
 	}
 	for (i = RCU_TORTURE_PIPE_LEN; i >= 0; i--) {
 		if (pipesummary[i] != 0)
@@ -2672,6 +2677,7 @@ rcu_torture_stats_print(void)
 	pr_cont("read-exits: %ld ", data_race(n_read_exits)); // Statistic.
 	pr_cont("nocb-toggles: %ld:%ld\n",
 		atomic_long_read(&n_nocb_offload), atomic_long_read(&n_nocb_deoffload));
+	pr_cont("gpwraps: %ld\n", n_gpwraps);
 
 	pr_alert("%s%s ", torture_type, TORTURE_FLAG);
 	if (atomic_read(&n_rcu_torture_mberror) ||
@@ -3842,6 +3848,58 @@ static int rcu_torture_preempt(void *unused)
 
 static enum cpuhp_state rcutor_hp;
 
+static struct hrtimer ovf_lag_timer;
+static bool ovf_lag_active;
+
+/* Timer handler for toggling RCU grace-period sequence overflow test lag value */
+static enum hrtimer_restart rcu_torture_ovf_lag_timer(struct hrtimer *timer)
+{
+	ktime_t next_delay;
+
+	if (ovf_lag_active) {
+		pr_alert("rcu-torture: Disabling ovf lag (value=0)\n");
+		rcu_set_torture_ovf_lag(0);
+		ovf_lag_active = false;
+		next_delay = ktime_set((ovf_cycle_mins - ovf_active_mins) * 60, 0);
+	} else {
+		pr_alert("rcu-torture: Enabling ovf lag (value=%d)\n", ovf_lag_gps);
+		rcu_set_torture_ovf_lag(ovf_lag_gps);
+		ovf_lag_active = true;
+		next_delay = ktime_set(ovf_active_mins * 60, 0);
+	}
+
+	if (torture_must_stop())
+		return HRTIMER_NORESTART;
+
+	hrtimer_forward_now(timer, next_delay);
+	return HRTIMER_RESTART;
+}
+
+static int rcu_torture_ovf_lag_init(void)
+{
+	if (ovf_cycle_mins <= 0 || ovf_active_mins <= 0) {
+		pr_alert("rcu-torture: lag timing parameters must be positive\n");
+		return -EINVAL;
+	}
+
+	hrtimer_init(&ovf_lag_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	ovf_lag_timer.function = rcu_torture_ovf_lag_timer;
+	ovf_lag_active = false;
+	hrtimer_start(&ovf_lag_timer,
+		      ktime_set((ovf_cycle_mins - ovf_active_mins) * 60, 0), HRTIMER_MODE_REL);
+
+	return 0;
+}
+
+static void rcu_torture_ovf_lag_cleanup(void)
+{
+	hrtimer_cancel(&ovf_lag_timer);
+
+	if (ovf_lag_active) {
+		rcu_set_torture_ovf_lag(0);
+		ovf_lag_active = false;
+	}
+}
 static void
 rcu_torture_cleanup(void)
 {
@@ -4015,6 +4073,8 @@ rcu_torture_cleanup(void)
 	torture_cleanup_end();
 	if (cur_ops->gp_slow_unregister)
 		cur_ops->gp_slow_unregister(NULL);
+
+	rcu_torture_ovf_lag_cleanup();
 }
 
 static void rcu_torture_leak_cb(struct rcu_head *rhp)
@@ -4508,6 +4568,10 @@ rcu_torture_init(void)
 	torture_init_end();
 	if (cur_ops->gp_slow_register && !WARN_ON_ONCE(!cur_ops->gp_slow_unregister))
 		cur_ops->gp_slow_register(&rcu_fwd_cb_nodelay);
+
+	if (rcu_torture_ovf_lag_init())
+		goto unwind;
+
 	return 0;
 
 unwind:
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b77ccc55557b..7b17b578956a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -80,6 +80,15 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *);
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
 	.gpwrap = true,
 };
+
+int rcu_get_gpwrap_count(int cpu)
+{
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+
+	return READ_ONCE(rdp->gpwrap_count);
+}
+EXPORT_SYMBOL_GPL(rcu_get_gpwrap_count);
+
 static struct rcu_state rcu_state = {
 	.level = { &rcu_state.node[0] },
 	.gp_state = RCU_GP_IDLE,
@@ -778,6 +787,25 @@ void rcu_request_urgent_qs_task(struct task_struct *t)
 	smp_store_release(per_cpu_ptr(&rcu_data.rcu_urgent_qs, cpu), true);
 }
 
+/**
+ * rcu_set_torture_ovf_lag - Set RCU GP sequence overflow lag value.
+ * @lag_gps: Set overflow lag to this many grace period worth of counters
+ * which is used by rcutorture to quickly force a gpwrap situation.
+ * @lag_gps = 0 means we reset it back to the boot-time value.
+ */
+static unsigned long seq_ovf_lag = ULONG_MAX / 4;
+
+void rcu_set_torture_ovf_lag(unsigned long lag_gps)
+{
+	unsigned long lag_seq_count;
+
+	lag_seq_count = (lag_gps == 0)
+			? ULONG_MAX / 4
+			: lag_gps << RCU_SEQ_CTR_SHIFT;
+	WRITE_ONCE(seq_ovf_lag, lag_seq_count);
+}
+EXPORT_SYMBOL_GPL(rcu_set_torture_ovf_lag);
+
 /*
  * When trying to report a quiescent state on behalf of some other CPU,
  * it is our responsibility to check for and handle potential overflow
@@ -788,9 +816,11 @@ void rcu_request_urgent_qs_task(struct task_struct *t)
 static void rcu_gpnum_ovf(struct rcu_node *rnp, struct rcu_data *rdp)
 {
 	raw_lockdep_assert_held_rcu_node(rnp);
-	if (ULONG_CMP_LT(rcu_seq_current(&rdp->gp_seq) + ULONG_MAX / 4,
-			 rnp->gp_seq))
+	if (ULONG_CMP_LT(rcu_seq_current(&rdp->gp_seq) + seq_ovf_lag,
+			 rnp->gp_seq)) {
 		WRITE_ONCE(rdp->gpwrap, true);
+		WRITE_ONCE(rdp->gpwrap_count, READ_ONCE(rdp->gpwrap_count) + 1);
+	}
 	if (ULONG_CMP_LT(rdp->rcu_iw_gp_seq + ULONG_MAX / 4, rnp->gp_seq))
 		rdp->rcu_iw_gp_seq = rnp->gp_seq + ULONG_MAX / 4;
 }
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index a9a811d9d7a3..63bea388c243 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -183,6 +183,7 @@ struct rcu_data {
 	bool		core_needs_qs;	/* Core waits for quiescent state. */
 	bool		beenonline;	/* CPU online at least once. */
 	bool		gpwrap;		/* Possible ->gp_seq wrap. */
+	unsigned int	gpwrap_count;	/* Count of GP sequence wrap. */
 	bool		cpu_started;	/* RCU watching this onlining CPU. */
 	struct rcu_node *mynode;	/* This CPU's leaf of hierarchy */
 	unsigned long grpmask;		/* Mask to apply to leaf qsmask. */
-- 
2.43.0

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