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Subject[PATCH tip/core/rcu 20/22] rcu: Move RCU grace-period cleanup into kthread
Date
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

As a first step towards allowing grace-period cleanup to be preemptible,
this commit moves the RCU grace-period cleanup into the same kthread
that is now used to initialize grace periods. This is needed to keep
scheduling latency down to a dull roar.

Reported-by: Mike Galbraith <mgalbraith@suse.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 112 ++++++++++++++++++++++++++++++------------------------
1 files changed, 62 insertions(+), 50 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index e5b4532..4a3f6b0 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1035,6 +1035,7 @@ rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
static int rcu_gp_kthread(void *arg)
{
unsigned long flags;
+ unsigned long gp_duration;
struct rcu_data *rdp;
struct rcu_node *rnp;
struct rcu_state *rsp = arg;
@@ -1125,6 +1126,65 @@ static int rcu_gp_kthread(void *arg)
rsp->fqs_state = RCU_SIGNAL_INIT;
raw_spin_unlock_irqrestore(&rnp->lock, flags);
put_online_cpus();
+
+ /* Handle grace-period end. */
+ rnp = rcu_get_root(rsp);
+ for (;;) {
+ wait_event_interruptible(rsp->gp_wq,
+ !ACCESS_ONCE(rnp->qsmask) &&
+ !rcu_preempt_blocked_readers_cgp(rnp));
+ if (!ACCESS_ONCE(rnp->qsmask) &&
+ !rcu_preempt_blocked_readers_cgp(rnp))
+ break;
+ flush_signals(current);
+ }
+
+ raw_spin_lock_irqsave(&rnp->lock, flags);
+ gp_duration = jiffies - rsp->gp_start;
+ if (gp_duration > rsp->gp_max)
+ rsp->gp_max = gp_duration;
+
+ /*
+ * We know the grace period is complete, but to everyone else
+ * it appears to still be ongoing. But it is also the case
+ * that to everyone else it looks like there is nothing that
+ * they can do to advance the grace period. It is therefore
+ * safe for us to drop the lock in order to mark the grace
+ * period as completed in all of the rcu_node structures.
+ *
+ * But if this CPU needs another grace period, it will take
+ * care of this while initializing the next grace period.
+ * We use RCU_WAIT_TAIL instead of the usual RCU_DONE_TAIL
+ * because the callbacks have not yet been advanced: Those
+ * callbacks are waiting on the grace period that just now
+ * completed.
+ */
+ if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
+ raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+
+ /*
+ * Propagate new ->completed value to rcu_node
+ * structures so that other CPUs don't have to
+ * wait until the start of the next grace period
+ * to process their callbacks.
+ */
+ rcu_for_each_node_breadth_first(rsp, rnp) {
+ /* irqs already disabled. */
+ raw_spin_lock(&rnp->lock);
+ rnp->completed = rsp->gpnum;
+ /* irqs remain disabled. */
+ raw_spin_unlock(&rnp->lock);
+ }
+ rnp = rcu_get_root(rsp);
+ raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+ }
+
+ rsp->completed = rsp->gpnum; /* Declare grace period done. */
+ trace_rcu_grace_period(rsp->name, rsp->completed, "end");
+ rsp->fqs_state = RCU_GP_IDLE;
+ if (cpu_needs_another_gp(rsp, rdp))
+ rsp->gp_flags = 1;
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
return 0;
}
@@ -1172,57 +1232,9 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
__releases(rcu_get_root(rsp)->lock)
{
- unsigned long gp_duration;
- struct rcu_node *rnp = rcu_get_root(rsp);
- struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
-
WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
-
- /*
- * Ensure that all grace-period and pre-grace-period activity
- * is seen before the assignment to rsp->completed.
- */
- smp_mb(); /* See above block comment. */
- gp_duration = jiffies - rsp->gp_start;
- if (gp_duration > rsp->gp_max)
- rsp->gp_max = gp_duration;
-
- /*
- * We know the grace period is complete, but to everyone else
- * it appears to still be ongoing. But it is also the case
- * that to everyone else it looks like there is nothing that
- * they can do to advance the grace period. It is therefore
- * safe for us to drop the lock in order to mark the grace
- * period as completed in all of the rcu_node structures.
- *
- * But if this CPU needs another grace period, it will take
- * care of this while initializing the next grace period.
- * We use RCU_WAIT_TAIL instead of the usual RCU_DONE_TAIL
- * because the callbacks have not yet been advanced: Those
- * callbacks are waiting on the grace period that just now
- * completed.
- */
- if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
-
- /*
- * Propagate new ->completed value to rcu_node structures
- * so that other CPUs don't have to wait until the start
- * of the next grace period to process their callbacks.
- */
- rcu_for_each_node_breadth_first(rsp, rnp) {
- raw_spin_lock(&rnp->lock); /* irqs already disabled. */
- rnp->completed = rsp->gpnum;
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
- }
- rnp = rcu_get_root(rsp);
- raw_spin_lock(&rnp->lock); /* irqs already disabled. */
- }
-
- rsp->completed = rsp->gpnum; /* Declare the grace period complete. */
- trace_rcu_grace_period(rsp->name, rsp->completed, "end");
- rsp->fqs_state = RCU_GP_IDLE;
- rcu_start_gp(rsp, flags); /* releases root node's rnp->lock. */
+ raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
+ wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */
}

/*
--
1.7.8


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