rcu: Avoid waking up CPUs having only kfree_rcu() callbacks

When CONFIG_RCU_FAST_NO_HZ is enabled, RCU will allow a given CPU to
enter dyntick-idle mode even if it still has RCU callbacks queued.
RCU avoids system hangs in this case by scheduling a timer for several
jiffies in the future.  However, if all of the callbacks on that CPU
are from kfree_rcu(), there is no reason to wake the CPU up, as it is
not a problem to defer freeing of memory.

This commit therefore tracks the number of callbacks on a given CPU
that are from kfree_rcu(), and avoids scheduling the timer if all of
a given CPU's callbacks are from kfree_rcu().

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

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index 8e00d461911e..4eb34fcc2a75 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -258,7 +258,7 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 
         /* If no RCU callbacks ready to invoke, just return. */
         if (&rcp->rcucblist == rcp->donetail) {
-                RCU_TRACE(trace_rcu_batch_start(rcp->name, 0, -1));
+                RCU_TRACE(trace_rcu_batch_start(rcp->name, 0, 0, -1));
                 RCU_TRACE(trace_rcu_batch_end(rcp->name, 0,
                                               ACCESS_ONCE(rcp->rcucblist),
                                               need_resched(),
@@ -269,7 +269,7 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
 
         /* Move the ready-to-invoke callbacks to a local list. */
         local_irq_save(flags);
-        RCU_TRACE(trace_rcu_batch_start(rcp->name, 0, -1));
+        RCU_TRACE(trace_rcu_batch_start(rcp->name, 0, rcp->qlen, -1));
         list = rcp->rcucblist;
         rcp->rcucblist = *rcp->donetail;
         *rcp->donetail = NULL;