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Subject[PATCH RFC tip/core/rcu 04/14] rcu/nocb: Print no-CBs diagnostics when rcutorture writer unduly delayed
Date
This commit causes locking, sleeping, and callback state to be printed
for no-CBs CPUs when the rcutorture writer is delayed sufficiently for
rcutorture to complain.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
kernel/rcu/rcutorture.c | 1 +
kernel/rcu/tree.h | 7 +++-
kernel/rcu/tree_plugin.h | 82 ++++++++++++++++++++++++++++++++++++++++
kernel/rcu/tree_stall.h | 5 +++
4 files changed, 94 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index b22947324423..3c9feca1eab1 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -2176,6 +2176,7 @@ rcu_torture_cleanup(void)
return;
}

+ show_rcu_gp_kthreads();
rcu_torture_barrier_cleanup();
torture_stop_kthread(rcu_torture_fwd_prog, fwd_prog_task);
torture_stop_kthread(rcu_torture_stall, stall_task);
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index e4df86db8137..c612f306fe89 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -212,7 +212,11 @@ struct rcu_data {
/* The following fields are used by GP kthread, hence own cacheline. */
raw_spinlock_t nocb_gp_lock ____cacheline_internodealigned_in_smp;
struct timer_list nocb_bypass_timer; /* Force nocb_bypass flush. */
- bool nocb_gp_sleep; /* Is the nocb GP thread asleep? */
+ u8 nocb_gp_sleep; /* Is the nocb GP thread asleep? */
+ u8 nocb_gp_bypass; /* Found a bypass on last scan? */
+ u8 nocb_gp_gp; /* GP to wait for on last scan? */
+ unsigned long nocb_gp_seq; /* If so, ->gp_seq to wait for. */
+ unsigned long nocb_gp_loops; /* # passes through wait code. */
struct swait_queue_head nocb_gp_wq; /* For nocb kthreads to sleep on. */
bool nocb_cb_sleep; /* Is the nocb CB thread asleep? */
struct task_struct *nocb_cb_kthread;
@@ -438,6 +442,7 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp);
static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
static void rcu_spawn_cpu_nocb_kthread(int cpu);
static void __init rcu_spawn_nocb_kthreads(void);
+static void show_rcu_nocb_state(struct rcu_data *rdp);
static void rcu_nocb_lock(struct rcu_data *rdp);
static void rcu_nocb_unlock(struct rcu_data *rdp);
static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index fcbd61738dff..aaeb8a658f4b 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2018,6 +2018,9 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
rcu_gp_kthread_wake();
}

+ my_rdp->nocb_gp_bypass = bypass;
+ my_rdp->nocb_gp_gp = needwait_gp;
+ my_rdp->nocb_gp_seq = needwait_gp ? wait_gp_seq : 0;
if (bypass && !rcu_nocb_poll) {
// At least one child with non-empty ->nocb_bypass, so set
// timer in order to avoid stranding its callbacks.
@@ -2052,6 +2055,7 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
WRITE_ONCE(my_rdp->nocb_gp_sleep, true);
raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
}
+ my_rdp->nocb_gp_seq = -1;
WARN_ON(signal_pending(current));
}

@@ -2068,6 +2072,7 @@ static int rcu_nocb_gp_kthread(void *arg)
struct rcu_data *rdp = arg;

for (;;) {
+ WRITE_ONCE(rdp->nocb_gp_loops, rdp->nocb_gp_loops + 1);
nocb_gp_wait(rdp);
cond_resched_tasks_rcu_qs();
}
@@ -2359,6 +2364,79 @@ void rcu_bind_current_to_nocb(void)
}
EXPORT_SYMBOL_GPL(rcu_bind_current_to_nocb);

+/*
+ * Dump out nocb grace-period kthread state for the specified rcu_data
+ * structure.
+ */
+static void show_rcu_nocb_gp_state(struct rcu_data *rdp)
+{
+ struct rcu_node *rnp = rdp->mynode;
+
+ pr_info("nocb GP %d %c%c%c%c%c%c %c[%c%c] %c%c:%ld rnp %d:%d %lu\n",
+ rdp->cpu,
+ "kK"[!!rdp->nocb_gp_kthread],
+ "lL"[raw_spin_is_locked(&rdp->nocb_gp_lock)],
+ "dD"[!!rdp->nocb_defer_wakeup],
+ "tT"[timer_pending(&rdp->nocb_timer)],
+ "bB"[timer_pending(&rdp->nocb_bypass_timer)],
+ "sS"[!!rdp->nocb_gp_sleep],
+ ".W"[swait_active(&rdp->nocb_gp_wq)],
+ ".W"[swait_active(&rnp->nocb_gp_wq[0])],
+ ".W"[swait_active(&rnp->nocb_gp_wq[1])],
+ ".B"[!!rdp->nocb_gp_bypass],
+ ".G"[!!rdp->nocb_gp_gp],
+ (long)rdp->nocb_gp_seq,
+ rnp->grplo, rnp->grphi, READ_ONCE(rdp->nocb_gp_loops));
+}
+
+/* Dump out nocb kthread state for the specified rcu_data structure. */
+static void show_rcu_nocb_state(struct rcu_data *rdp)
+{
+ struct rcu_segcblist *rsclp = &rdp->cblist;
+ bool waslocked;
+ bool wastimer;
+ bool wassleep;
+
+ if (rdp->nocb_gp_rdp == rdp)
+ show_rcu_nocb_gp_state(rdp);
+
+ pr_info(" CB %d->%d %c%c%c%c%c%c F%ld L%ld C%d %c%c%c%c%c q%ld\n",
+ rdp->cpu, rdp->nocb_gp_rdp->cpu,
+ "kK"[!!rdp->nocb_cb_kthread],
+ "bB"[raw_spin_is_locked(&rdp->nocb_bypass_lock)],
+ "cC"[!!atomic_read(&rdp->nocb_lock_contended)],
+ "lL"[raw_spin_is_locked(&rdp->nocb_lock)],
+ "sS"[!!rdp->nocb_cb_sleep],
+ ".W"[swait_active(&rdp->nocb_cb_wq)],
+ jiffies - rdp->nocb_bypass_first,
+ jiffies - rdp->nocb_nobypass_last,
+ rdp->nocb_nobypass_count,
+ ".D"[rcu_segcblist_ready_cbs(rsclp)],
+ ".W"[!rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL)],
+ ".R"[!rcu_segcblist_restempty(rsclp, RCU_WAIT_TAIL)],
+ ".N"[!rcu_segcblist_restempty(rsclp, RCU_NEXT_READY_TAIL)],
+ ".B"[!!rcu_cblist_n_cbs(&rdp->nocb_bypass)],
+ rcu_segcblist_n_cbs(&rdp->cblist));
+
+ /* It is OK for GP kthreads to have GP state. */
+ if (rdp->nocb_gp_rdp == rdp)
+ return;
+
+ waslocked = raw_spin_is_locked(&rdp->nocb_gp_lock);
+ wastimer = timer_pending(&rdp->nocb_timer);
+ wassleep = swait_active(&rdp->nocb_gp_wq);
+ if (!rdp->nocb_defer_wakeup && !rdp->nocb_gp_sleep &&
+ !waslocked && !wastimer && !wassleep)
+ return; /* Nothing untowards. */
+
+ pr_info(" !!! %c%c%c%c %c\n",
+ "lL"[waslocked],
+ "dD"[!!rdp->nocb_defer_wakeup],
+ "tT"[wastimer],
+ "sS"[!!rdp->nocb_gp_sleep],
+ ".W"[wassleep]);
+}
+
#else /* #ifdef CONFIG_RCU_NOCB_CPU */

/* No ->nocb_lock to acquire. */
@@ -2436,6 +2514,10 @@ static void __init rcu_spawn_nocb_kthreads(void)
{
}

+static void show_rcu_nocb_state(struct rcu_data *rdp)
+{
+}
+
#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */

/*
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 0627a66699a6..841ab43f3e60 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -589,6 +589,11 @@ void show_rcu_gp_kthreads(void)
cpu, (long)rdp->gp_seq_needed);
}
}
+ for_each_possible_cpu(cpu) {
+ rdp = per_cpu_ptr(&rcu_data, cpu);
+ if (rcu_segcblist_is_offloaded(&rdp->cblist))
+ show_rcu_nocb_state(rdp);
+ }
/* sched_show_task(rcu_state.gp_kthread); */
}
EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
--
2.17.1
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