[tip: core/rcu] rcu: Force tick on for nohz_full CPUs not reaching quiescent states

From: tip-bot2 for Paul E. McKenney
Date: Thu Oct 31 2019 - 07:57:34 EST

The following commit has been merged into the core/rcu branch of tip:

Commit-ID: 66e4c33b51bc515ca803c0948cf1525b53ffd631
Gitweb: https://git.kernel.org/tip/66e4c33b51bc515ca803c0948cf1525b53ffd631
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate: Mon, 12 Aug 2019 16:14:00 -07:00
Committer: Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 28 Oct 2019 07:02:21 -07:00

rcu: Force tick on for nohz_full CPUs not reaching quiescent states

CPUs running for long time periods in the kernel in nohz_full mode
might leave the scheduling-clock interrupt disabled for then full
duration of their in-kernel execution. This can (among other things)
delay grace periods. This commit therefore forces the tick back on
for any nohz_full CPU that is failing to pass through a quiescent state
upon return from interrupt, which the resched_cpu() will induce.

Reported-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
[ paulmck: Clear ->rcu_forced_tick as reported by Joel Fernandes testing. ]
[ paulmck: Apply Joel Fernandes TICK_DEP_MASK_RCU->TICK_DEP_BIT_RCU fix. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
kernel/rcu/tree.c | 38 +++++++++++++++++++++++++++++++-------
kernel/rcu/tree.h | 1 +
2 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 66354ef..9fda338 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -651,6 +651,12 @@ static __always_inline void rcu_nmi_exit_common(bool irq)
*/
if (rdp->dynticks_nmi_nesting != 1) {
trace_rcu_dyntick(TPS("--="), rdp->dynticks_nmi_nesting, rdp->dynticks_nmi_nesting - 2, rdp->dynticks);
+ if (tick_nohz_full_cpu(rdp->cpu) &&
+ rdp->dynticks_nmi_nesting == 2 &&
+ rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) {
+ rdp->rcu_forced_tick = true;
+ tick_dep_set_cpu(rdp->cpu, TICK_DEP_MASK_RCU);
+ }
WRITE_ONCE(rdp->dynticks_nmi_nesting, /* No store tearing. */
rdp->dynticks_nmi_nesting - 2);
return;
@@ -886,6 +892,18 @@ void rcu_irq_enter_irqson(void)
local_irq_restore(flags);
}

+/*
+ * If the scheduler-clock interrupt was enabled on a nohz_full CPU
+ * in order to get to a quiescent state, disable it.
+ */
+void rcu_disable_tick_upon_qs(struct rcu_data *rdp)
+{
+ if (tick_nohz_full_cpu(rdp->cpu) && rdp->rcu_forced_tick) {
+ tick_dep_clear_cpu(rdp->cpu, TICK_DEP_BIT_RCU);
+ rdp->rcu_forced_tick = false;
+ }
+}
+
/**
* rcu_is_watching - see if RCU thinks that the current CPU is not idle
*
@@ -1980,6 +1998,7 @@ rcu_report_qs_rdp(int cpu, struct rcu_data *rdp)
if (!offloaded)
needwake = rcu_accelerate_cbs(rnp, rdp);

+ rcu_disable_tick_upon_qs(rdp);
rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
/* ^^^ Released rnp->lock */
if (needwake)
@@ -2265,6 +2284,7 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
int cpu;
unsigned long flags;
unsigned long mask;
+ struct rcu_data *rdp;
struct rcu_node *rnp;

rcu_for_each_leaf_node(rnp) {
@@ -2289,8 +2309,11 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
for_each_leaf_node_possible_cpu(rnp, cpu) {
unsigned long bit = leaf_node_cpu_bit(rnp, cpu);
if ((rnp->qsmask & bit) != 0) {
- if (f(per_cpu_ptr(&rcu_data, cpu)))
+ rdp = per_cpu_ptr(&rcu_data, cpu);
+ if (f(rdp)) {
mask |= bit;
+ rcu_disable_tick_upon_qs(rdp);
+ }
}
}
if (mask != 0) {
@@ -2318,7 +2341,7 @@ void rcu_force_quiescent_state(void)
rnp = __this_cpu_read(rcu_data.mynode);
for (; rnp != NULL; rnp = rnp->parent) {
ret = (READ_ONCE(rcu_state.gp_flags) & RCU_GP_FLAG_FQS) ||
- !raw_spin_trylock(&rnp->fqslock);
+ !raw_spin_trylock(&rnp->fqslock);
if (rnp_old != NULL)
raw_spin_unlock(&rnp_old->fqslock);
if (ret)
@@ -2851,7 +2874,7 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
{
if (atomic_dec_and_test(&rcu_state.barrier_cpu_count)) {
rcu_barrier_trace(TPS("LastCB"), -1,
- rcu_state.barrier_sequence);
+ rcu_state.barrier_sequence);
complete(&rcu_state.barrier_completion);
} else {
rcu_barrier_trace(TPS("CB"), -1, rcu_state.barrier_sequence);
@@ -2875,7 +2898,7 @@ static void rcu_barrier_func(void *unused)
} else {
debug_rcu_head_unqueue(&rdp->barrier_head);
rcu_barrier_trace(TPS("IRQNQ"), -1,
- rcu_state.barrier_sequence);
+ rcu_state.barrier_sequence);
}
rcu_nocb_unlock(rdp);
}
@@ -2902,7 +2925,7 @@ void rcu_barrier(void)
/* Did someone else do our work for us? */
if (rcu_seq_done(&rcu_state.barrier_sequence, s)) {
rcu_barrier_trace(TPS("EarlyExit"), -1,
- rcu_state.barrier_sequence);
+ rcu_state.barrier_sequence);
smp_mb(); /* caller's subsequent code after above check. */
mutex_unlock(&rcu_state.barrier_mutex);
return;
@@ -2934,11 +2957,11 @@ void rcu_barrier(void)
continue;
if (rcu_segcblist_n_cbs(&rdp->cblist)) {
rcu_barrier_trace(TPS("OnlineQ"), cpu,
- rcu_state.barrier_sequence);
+ rcu_state.barrier_sequence);
smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
} else {
rcu_barrier_trace(TPS("OnlineNQ"), cpu,
- rcu_state.barrier_sequence);
+ rcu_state.barrier_sequence);
}
}
put_online_cpus();
@@ -3160,6 +3183,7 @@ void rcu_cpu_starting(unsigned int cpu)
rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags);
if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */
+ rcu_disable_tick_upon_qs(rdp);
/* Report QS -after- changing ->qsmaskinitnext! */
rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
} else {
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index c612f30..055c317 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -181,6 +181,7 @@ struct rcu_data {
atomic_t dynticks; /* Even value for idle, else odd. */
bool rcu_need_heavy_qs; /* GP old, so heavy quiescent state! */
bool rcu_urgent_qs; /* GP old need light quiescent state. */
+ bool rcu_forced_tick; /* Forced tick to provide QS. */
#ifdef CONFIG_RCU_FAST_NO_HZ
bool all_lazy; /* All CPU's CBs lazy at idle start? */
unsigned long last_accelerate; /* Last jiffy CBs were accelerated. */