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rcu: Prevent force_quiescent_state() memory contention 

Large systems running RCU_FAST_NO_HZ kernels see extreme memory
contention on the rcu_state structure's ->fqslock field.  This
can be avoided by disabling RCU_FAST_NO_HZ, either at compile time
or at boot time (via the nohz kernel boot parameter), but large
systems will no doubt become sensitive to energy consumption.
This commit therefore uses a combining-tree approach to spread the
memory contention across new cache lines in the leaf rcu_node structures.
This can be thought of as a tournament lock that has only a try-lock
acquisition primitive.

The effect on small systems is minimal, because such systems have
an rcu_node "tree" consisting of a single node.  In addition, this
functionality is not used on fastpaths.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Paul E. McKenney 2012-06-26 17:00:35 -07:00
parent 4605c0143c
commit 394f2769aa
2 changed files with 38 additions and 10 deletions
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47
kernel/rcutree.c
View File

@ -61,6 +61,7 @@
/* Data structures. */
static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
#define RCU_STATE_INITIALIZER(sname, cr) { \
.level = { &sname##_state.node[0] }, \
@ -1807,16 +1808,35 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
static void force_quiescent_state(struct rcu_state *rsp)
{
unsigned long flags;
struct rcu_node *rnp = rcu_get_root(rsp);
bool ret;
struct rcu_node *rnp;
struct rcu_node *rnp_old = NULL;
if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS)
/* Funnel through hierarchy to reduce memory contention. */
rnp = per_cpu_ptr(rsp->rda, raw_smp_processor_id())->mynode;
for (; rnp != NULL; rnp = rnp->parent) {
ret = (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) ||
!raw_spin_trylock(&rnp->fqslock);
if (rnp_old != NULL)
raw_spin_unlock(&rnp_old->fqslock);
if (ret) {
rsp->n_force_qs_lh++;
return;
}
rnp_old = rnp;
}
/* rnp_old == rcu_get_root(rsp), rnp == NULL. */
/* Reached the root of the rcu_node tree, acquire lock. */
raw_spin_lock_irqsave(&rnp_old->lock, flags);
raw_spin_unlock(&rnp_old->fqslock);
if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
rsp->n_force_qs_lh++;
raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
return; /* Someone beat us to it. */
if (!raw_spin_trylock_irqsave(&rnp->lock, flags)) {
rsp->n_force_qs_lh++; /* Inexact, can lose counts. Tough! */
return;
}
rsp->gp_flags |= RCU_GP_FLAG_FQS;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */
}
@ -2704,10 +2724,14 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp)
static void __init rcu_init_one(struct rcu_state *rsp,
struct rcu_data __percpu *rda)
{
static char *buf[] = { "rcu_node_level_0",
"rcu_node_level_1",
"rcu_node_level_2",
"rcu_node_level_3" }; /* Match MAX_RCU_LVLS */
static char *buf[] = { "rcu_node_0",
"rcu_node_1",
"rcu_node_2",
"rcu_node_3" }; /* Match MAX_RCU_LVLS */
static char *fqs[] = { "rcu_node_fqs_0",
"rcu_node_fqs_1",
"rcu_node_fqs_2",
"rcu_node_fqs_3" }; /* Match MAX_RCU_LVLS */
int cpustride = 1;
int i;
int j;
@ -2732,6 +2756,9 @@ static void __init rcu_init_one(struct rcu_state *rsp,
raw_spin_lock_init(&rnp->lock);
lockdep_set_class_and_name(&rnp->lock,
&rcu_node_class[i], buf[i]);
raw_spin_lock_init(&rnp->fqslock);
lockdep_set_class_and_name(&rnp->fqslock,
&rcu_fqs_class[i], fqs[i]);
rnp->gpnum = 0;
rnp->qsmask = 0;
rnp->qsmaskinit = 0;

1
kernel/rcutree.h
View File

@ -202,6 +202,7 @@ struct rcu_node {
/* per-CPU kthreads as needed. */
unsigned int node_kthread_status;
/* State of node_kthread_task for tracing. */
raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
} ____cacheline_internodealigned_in_smp;
/*