rcu: Kick CPU halfway to RCU CPU stall warning

When an RCU CPU stall warning occurs, the CPU invokes resched_cpu() on
itself.  This can help move the grace period forward in some situations,
but it would be even better to do this -before- the RCU CPU stall warning.
This commit therefore causes resched_cpu() to be called every five jiffies
once the system is halfway to an RCU CPU stall warning.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 25 insertions, 1 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index dd081987a8ec..5243ebea0fc1 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -755,6 +755,12 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp,
 }
 
 /*
+ * This function really isn't for public consumption, but RCU is special in
+ * that context switches can allow the state machine to make progress.
+ */
+extern void resched_cpu(int cpu);
+
+/*
  * Return true if the specified CPU has passed through a quiescent
  * state by virtue of being in or having passed through an dynticks
  * idle state since the last call to dyntick_save_progress_counter()
@@ -812,16 +818,34 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
          */
         rcu_kick_nohz_cpu(rdp->cpu);
 
+        /*
+         * Alternatively, the CPU might be running in the kernel
+         * for an extended period of time without a quiescent state.
+         * Attempt to force the CPU through the scheduler to gain the
+         * needed quiescent state, but only if the grace period has gone
+         * on for an uncommonly long time.  If there are many stuck CPUs,
+         * we will beat on the first one until it gets unstuck, then move
+         * to the next.  Only do this for the primary flavor of RCU.
+         */
+        if (rdp->rsp == rcu_state &&
+            ULONG_CMP_GE(ACCESS_ONCE(jiffies), rdp->rsp->jiffies_resched)) {
+                rdp->rsp->jiffies_resched += 5;
+                resched_cpu(rdp->cpu);
+        }
+
         return 0;
 }
 
 static void record_gp_stall_check_time(struct rcu_state *rsp)
 {
         unsigned long j = ACCESS_ONCE(jiffies);
+        unsigned long j1;
 
         rsp->gp_start = j;
         smp_wmb(); /* Record start time before stall time. */
-        rsp->jiffies_stall = j + rcu_jiffies_till_stall_check();
+        j1 = rcu_jiffies_till_stall_check();
+        rsp->jiffies_stall = j + j1;
+        rsp->jiffies_resched = j + j1 / 2;
 }
 
 /*