rcu: Make force_quiescent_state() start grace period if needed

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>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <12626465501854-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 13 insertions, 0 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d9202857d3ad..55e8f6ef8195 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 edb6fae0fa94..bd5d78ad1c48 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. */