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rcu: Get rid of synchronize_sched_expedited()'s polling loop

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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>
This commit is contained in:
Paul E. McKenney
2015-06-24 14:20:08 -07:00
parent d6ada2cf2f
commit 385b73c06f
3 changed files with 47 additions and 59 deletions
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95
kernel/rcu/tree.c
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@@ -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, \
}
@@ -3272,6 +3272,22 @@ 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
*
@@ -3286,15 +3302,15 @@ 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. */
@@ -3310,60 +3326,25 @@ 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);
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;
}
/* 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);
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. */
@@ -3383,7 +3364,7 @@ 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();
@@ -3940,6 +3921,7 @@ 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. */
@@ -3998,6 +3980,9 @@ 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]);
}
}