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Subject[PATCH tip/core/rcu 10/11] rcu: make force_quiescent_state() start grace period if needed
Date
From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Grace periods cannot be started while force_quiescent_state() is
active. This is OK in that the affected CPUs will try again later,
but it does induce needless grace-period delays. This patch causes
rcu_start_gp() to record a failed attempt to start a grace period.
When force_quiescent_state() prepares to return, it then starts the
grace period if there was such a failed attempt.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree.c | 8 ++++++++
kernel/rcutree.h | 5 +++++
2 files changed, 13 insertions(+), 0 deletions(-)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d920285..55e8f6e 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -660,6 +660,8 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
struct rcu_node *rnp = rcu_get_root(rsp);

if (!cpu_needs_another_gp(rsp, rdp) || rsp->fqs_active) {
+ if (cpu_needs_another_gp(rsp, rdp))
+ rsp->fqs_need_gp = 1;
if (rnp->completed == rsp->completed) {
spin_unlock_irqrestore(&rnp->lock, flags);
return;
@@ -1239,6 +1241,12 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
break;
}
rsp->fqs_active = 0;
+ if (rsp->fqs_need_gp) {
+ spin_unlock(&rsp->fqslock); /* irqs remain disabled */
+ rsp->fqs_need_gp = 0;
+ rcu_start_gp(rsp, flags); /* releases rnp->lock */
+ return;
+ }
spin_unlock(&rnp->lock); /* irqs remain disabled */
unlock_fqs_ret:
spin_unlock_irqrestore(&rsp->fqslock, flags);
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index edb6fae..bd5d78a 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -278,6 +278,11 @@ struct rcu_state {
/* Force QS state. */
u8 fqs_active; /* force_quiescent_state() */
/* is running. */
+ u8 fqs_need_gp; /* A CPU was prevented from */
+ /* starting a new grace */
+ /* period because */
+ /* force_quiescent_state() */
+ /* was running. */
long gpnum; /* Current gp number. */
long completed; /* # of last completed gp. */

--
1.5.2.5


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