[PATCH RFC tip/core/rcu 03/14] rcu: Get rid of synchronize_sched_expedited()'s polling loop

Paul E. McKenney
2015-06-30T22:28:20Z

This commit gets rid of synchronize_sched_expedited()'s mutex_trylock()
polling loop in favor of a funnel-locking scheme based on the rcu_node
tree. The work-done check is done at each level of the tree, allowing
high-contention situations to be resolved quickly with reasonable levels
of mutex contention.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcu/tree.c | 95 +++++++++++++++++++++----------------------------
kernel/rcu/tree.h | 8 +++--
kernel/rcu/tree_trace.c | 3 +-
3 files changed, 47 insertions(+), 59 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 68212d1bfaef..887370b7e52a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -70,6 +70,7 @@ MODULE_ALIAS("rcutree");

static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
+static struct lock_class_key rcu_exp_class[RCU_NUM_LVLS];

/*
* In order to export the rcu_state name to the tracing tools, it
@@ -103,7 +104,6 @@ struct rcu_state sname##_state = { \
.orphan_nxttail = &sname##_state.orphan_nxtlist, \
.orphan_donetail = &sname##_state.orphan_donelist, \
.barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
- .expedited_mutex = __MUTEX_INITIALIZER(sname##_state.expedited_mutex), \
.name = RCU_STATE_NAME(sname), \
.abbr = sabbr, \
}
@@ -3324,6 +3324,22 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
return 0;
}

+/* Common code for synchronize_sched_expedited() work-done checking. */
+static bool sync_sched_exp_wd(struct rcu_state *rsp, struct rcu_node *rnp,
+ atomic_long_t *stat, unsigned long s)
+{
+ if (ULONG_CMP_GE(READ_ONCE(rsp->expedited_sequence), s)) {
+ if (rnp)
+ mutex_unlock(&rnp->exp_funnel_mutex);
+ /* Ensure test happens before caller kfree(). */
+ smp_mb__before_atomic(); /* ^^^ */
+ atomic_long_inc(stat);
+ put_online_cpus();
+ return true;
+ }
+ return false;
+}
+
/**
* synchronize_sched_expedited - Brute-force RCU-sched grace period
*
@@ -3338,15 +3354,15 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
* This implementation can be thought of as an application of sequence
* locking to expedited grace periods, but using the sequence counter to
* determine when someone else has already done the work instead of for
- * retrying readers. We do a mutex_trylock() polling loop, but if we fail
- * too many times in a row, we fall back to synchronize_sched().
+ * retrying readers.
*/
void synchronize_sched_expedited(void)
{
int cpu;
long s;
- int trycount = 0;
struct rcu_state *rsp = &rcu_sched_state;
+ struct rcu_node *rnp0;
+ struct rcu_node *rnp1 = NULL;

/* Take a snapshot of the sequence number. */
smp_mb(); /* Caller's modifications seen first by other CPUs. */
@@ -3362,60 +3378,25 @@ void synchronize_sched_expedited(void)
WARN_ON_ONCE(cpu_is_offline(raw_smp_processor_id()));

/*
- * Each pass through the following loop attempts to acquire
- * ->expedited_mutex, checking for others doing our work each time.
+ * Each pass through the following loop works its way
+ * up the rcu_node tree, returning if others have done the
+ * work or otherwise falls through holding the root rnp's
+ * ->exp_funnel_mutex. The mapping from CPU to rcu_node structure
+ * can be inexact, as it is just promoting locality and is not
+ * strictly needed for correctness.
*/
- while (!mutex_trylock(&rsp->expedited_mutex)) {
- put_online_cpus();
- atomic_long_inc(&rsp->expedited_tryfail);
-
- /* Check to see if someone else did our work for us. */
- if (ULONG_CMP_GE(READ_ONCE(rsp->expedited_sequence), s)) {
- /* ensure test happens before caller kfree */
- smp_mb__before_atomic(); /* ^^^ */
- atomic_long_inc(&rsp->expedited_workdone1);
- return;
- }
-
- /* No joy, try again later. Or just synchronize_sched(). */
- if (trycount++ < 10) {
- udelay(trycount * num_online_cpus());
- } else {
- wait_rcu_gp(call_rcu_sched);
- atomic_long_inc(&rsp->expedited_normal);
+ rnp0 = per_cpu_ptr(rsp->rda, raw_smp_processor_id())->mynode;
+ for (; rnp0 != NULL; rnp0 = rnp0->parent) {
+ if (sync_sched_exp_wd(rsp, rnp1, &rsp->expedited_workdone1, s))
return;
- }
-
- /* Recheck to see if someone else did our work for us. */
- if (ULONG_CMP_GE(READ_ONCE(rsp->expedited_sequence), s)) {
- /* ensure test happens before caller kfree */
- smp_mb__before_atomic(); /* ^^^ */
- atomic_long_inc(&rsp->expedited_workdone2);
- return;
- }
-
- /*
- * Refetching sync_sched_expedited_started allows later
- * callers to piggyback on our grace period. We retry
- * after they started, so our grace period works for them,
- * and they started after our first try, so their grace
- * period works for us.
- */
- if (!try_get_online_cpus()) {
- /* CPU hotplug operation in flight, use normal GP. */
- wait_rcu_gp(call_rcu_sched);
- atomic_long_inc(&rsp->expedited_normal);
- return;
- }
+ mutex_lock(&rnp0->exp_funnel_mutex);
+ if (rnp1)
+ mutex_unlock(&rnp1->exp_funnel_mutex);
+ rnp1 = rnp0;
}
-
- /* Recheck yet again to see if someone else did our work for us. */
- if (ULONG_CMP_GE(READ_ONCE(rsp->expedited_sequence), s)) {
- rsp->expedited_workdone3++;
- mutex_unlock(&rsp->expedited_mutex);
- smp_mb(); /* ensure test happens before caller kfree */
+ rnp0 = rnp1; /* rcu_get_root(rsp), AKA root rcu_node structure. */
+ if (sync_sched_exp_wd(rsp, rnp0, &rsp->expedited_workdone2, s))
return;
- }

WRITE_ONCE(rsp->expedited_sequence, rsp->expedited_sequence + 1);
smp_mb(); /* Ensure expedited GP seen after counter increment. */
@@ -3435,7 +3416,7 @@ void synchronize_sched_expedited(void)
smp_mb(); /* Ensure expedited GP seen before counter increment. */
WRITE_ONCE(rsp->expedited_sequence, rsp->expedited_sequence + 1);
WARN_ON_ONCE(rsp->expedited_sequence & 0x1);
- mutex_unlock(&rsp->expedited_mutex);
+ mutex_unlock(&rnp0->exp_funnel_mutex);
smp_mb(); /* ensure subsequent action seen after grace period. */

put_online_cpus();
@@ -3992,6 +3973,7 @@ static void __init rcu_init_one(struct rcu_state *rsp,
{
static const char * const buf[] = RCU_NODE_NAME_INIT;
static const char * const fqs[] = RCU_FQS_NAME_INIT;
+ static const char * const exp[] = RCU_EXP_NAME_INIT;
static u8 fl_mask = 0x1;

int levelcnt[RCU_NUM_LVLS]; /* # nodes in each level. */
@@ -4050,6 +4032,9 @@ static void __init rcu_init_one(struct rcu_state *rsp,
rnp->level = i;
INIT_LIST_HEAD(&rnp->blkd_tasks);
rcu_init_one_nocb(rnp);
+ mutex_init(&rnp->exp_funnel_mutex);
+ lockdep_set_class_and_name(&rnp->exp_funnel_mutex,
+ &rcu_exp_class[i], exp[i]);
}
}

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index f15dd1e08ddd..f0f4dd96dd73 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -68,6 +68,7 @@
# define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 }
# define RCU_NODE_NAME_INIT { "rcu_node_0" }
# define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" }
+# define RCU_EXP_NAME_INIT { "rcu_node_exp_0" }
#elif NR_CPUS <= RCU_FANOUT_2
# define RCU_NUM_LVLS 2
# define NUM_RCU_LVL_0 1
@@ -76,6 +77,7 @@
# define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
# define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" }
# define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" }
+# define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" }
#elif NR_CPUS <= RCU_FANOUT_3
# define RCU_NUM_LVLS 3
# define NUM_RCU_LVL_0 1
@@ -85,6 +87,7 @@
# define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
# define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
# define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
+# define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
#elif NR_CPUS <= RCU_FANOUT_4
# define RCU_NUM_LVLS 4
# define NUM_RCU_LVL_0 1
@@ -95,6 +98,7 @@
# define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
# define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
# define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
+# define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
#else
# error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
#endif /* #if (NR_CPUS) <= RCU_FANOUT_1 */
@@ -237,6 +241,8 @@ struct rcu_node {
int need_future_gp[2];
/* Counts of upcoming no-CB GP requests. */
raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
+
+ struct mutex exp_funnel_mutex ____cacheline_internodealigned_in_smp;
} ____cacheline_internodealigned_in_smp;

/*
@@ -478,12 +484,10 @@ struct rcu_state {
/* _rcu_barrier(). */
/* End of fields guarded by barrier_mutex. */

- struct mutex expedited_mutex; /* Serializes expediting. */
unsigned long expedited_sequence; /* Take a ticket. */
atomic_long_t expedited_tryfail; /* # acquisition failures. */
atomic_long_t expedited_workdone1; /* # done by others #1. */
atomic_long_t expedited_workdone2; /* # done by others #2. */
- unsigned long expedited_workdone3; /* # done by others #3. */
atomic_long_t expedited_normal; /* # fallbacks to normal. */

unsigned long jiffies_force_qs; /* Time at which to invoke */
diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c
index a1ab3a5f6290..d2aab8dcd58e 100644
--- a/kernel/rcu/tree_trace.c
+++ b/kernel/rcu/tree_trace.c
@@ -185,12 +185,11 @@ static int show_rcuexp(struct seq_file *m, void *v)
{
struct rcu_state *rsp = (struct rcu_state *)m->private;

- seq_printf(m, "t=%lu tf=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu sc=%lu\n",
+ seq_printf(m, "t=%lu tf=%lu wd1=%lu wd2=%lu n=%lu sc=%lu\n",
rsp->expedited_sequence,
atomic_long_read(&rsp->expedited_tryfail),
atomic_long_read(&rsp->expedited_workdone1),
atomic_long_read(&rsp->expedited_workdone2),
- rsp->expedited_workdone3,
atomic_long_read(&rsp->expedited_normal),
rsp->expedited_sequence / 2);
return 0;
--
1.8.1.5