[PATCH RFC tip/core/rcu 13/14] rcu: Add stall warnings to synchronize_sched_expedited()

Paul E. McKenney
2015-06-30T22:27:41Z

Although synchronize_sched_expedited() historically has no RCU CPU stall
warnings, the availability of the rcupdate.rcu_expedited boot parameter
invalidates the old assumption that synchronize_sched()'s stall warnings
would suffice. This commit therefore adds RCU CPU stall warnings to
synchronize_sched_expedited().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcu/tree.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++++++----
kernel/rcu/tree.h | 1 +
2 files changed, 55 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f39ac217916a..4992bfd360b6 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3421,16 +3421,65 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
return rnp1;
}

+/* Invoked on each online non-idle CPU for expedited quiescent state. */
static int synchronize_sched_expedited_cpu_stop(void *data)
{
- struct rcu_state *rsp = data;
+ struct rcu_data *rdp = data;
+ struct rcu_state *rsp = rdp->rsp;

/* We are here: If we are last, do the wakeup. */
+ rdp->exp_done = true;
if (atomic_dec_and_test(&rsp->expedited_need_qs))
wake_up(&rsp->expedited_wq);
return 0;
}

+static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
+{
+ int cpu;
+ unsigned long jiffies_stall;
+ unsigned long jiffies_start;
+ struct rcu_data *rdp;
+ int ret;
+
+ jiffies_stall = rcu_jiffies_till_stall_check();
+ jiffies_start = jiffies;
+
+ for (;;) {
+ ret = wait_event_interruptible_timeout(
+ rsp->expedited_wq,
+ !atomic_read(&rsp->expedited_need_qs),
+ jiffies_stall);
+ if (ret > 0)
+ return;
+ if (ret < 0) {
+ /* Hit a signal, disable CPU stall warnings. */
+ wait_event(rsp->expedited_wq,
+ !atomic_read(&rsp->expedited_need_qs));
+ return;
+ }
+ pr_err("INFO: %s detected expedited stalls on CPUs: {",
+ rsp->name);
+ for_each_online_cpu(cpu) {
+ rdp = per_cpu_ptr(rsp->rda, cpu);
+
+ if (rdp->exp_done)
+ continue;
+ pr_cont(" %d", cpu);
+ }
+ pr_cont(" } %lu jiffies s: %lu\n",
+ jiffies - jiffies_start, rsp->expedited_sequence);
+ for_each_online_cpu(cpu) {
+ rdp = per_cpu_ptr(rsp->rda, cpu);
+
+ if (rdp->exp_done)
+ continue;
+ dump_cpu_task(cpu);
+ }
+ jiffies_stall = 3 * rcu_jiffies_till_stall_check() + 3;
+ }
+}
+
/**
* synchronize_sched_expedited - Brute-force RCU-sched grace period
*
@@ -3480,19 +3529,20 @@ void synchronize_sched_expedited(void)
struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);

+ rdp->exp_done = false;
+
/* Skip our CPU and any idle CPUs. */
if (raw_smp_processor_id() == cpu ||
!(atomic_add_return(0, &rdtp->dynticks) & 0x1))
continue;
atomic_inc(&rsp->expedited_need_qs);
stop_one_cpu_nowait(cpu, synchronize_sched_expedited_cpu_stop,
- rsp, &rdp->exp_stop_work);
+ rdp, &rdp->exp_stop_work);
}

/* Remove extra count and, if necessary, wait for CPUs to stop. */
if (!atomic_dec_and_test(&rsp->expedited_need_qs))
- wait_event(rsp->expedited_wq,
- !atomic_read(&rsp->expedited_need_qs));
+ synchronize_sched_expedited_wait(rsp);

rcu_exp_gp_seq_end(rsp);
mutex_unlock(&rnp->exp_funnel_mutex);
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 1b38f11dba06..cb402b5c2e71 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -370,6 +370,7 @@ struct rcu_data {
struct rcu_head oom_head;
#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
struct mutex exp_funnel_mutex;
+ bool exp_done; /* Expedited QS for this CPU? */

/* 7) Callback offloading. */
#ifdef CONFIG_RCU_NOCB_CPU
--
1.8.1.5