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, 1 deletions

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index fddff92d6676..af2af3cc5e65 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -265,7 +265,8 @@ struct rcu_data {
          */
         struct rcu_head *nxtlist;
         struct rcu_head **nxttail[RCU_NEXT_SIZE];
-        long            qlen;           /* # of queued callbacks */
+        long            qlen_lazy;      /* # of lazy queued callbacks */
+        long            qlen;           /* # of queued callbacks, incl lazy */
         long            qlen_last_fqs_check;
                                         /* qlen at last check for QS forcing */
         unsigned long   n_cbs_invoked;  /* count of RCU cbs invoked. */