rcu: Hold off RCU_FAST_NO_HZ after timer posted

This commit handles workloads that transition quickly between idle and
non-idle, and where the CPU's callbacks cannot be invoked, but where
RCU does not have anything immediate for the CPU to do.  Without this
patch, the RCU_FAST_NO_HZ code can be invoked repeatedly on each entry
to idle.  The commit sets the per-CPU rcu_dyntick_holdoff variable to
hold off further attempts for a tick.

Reported-by: "Abou Gazala, Neven M" <neven.m.abou.gazala@intel.com>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 392a65136a72abe8225606e9d6f2777aae20c6e6..c023464816bede2d7e5ec6f85b47d56b959119d1 100644 (file)
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2138,7 +2138,7 @@ static void rcu_prepare_for_idle(int cpu)
                /* Can we go dyntick-idle despite still having callbacks? */
                trace_rcu_prep_idle("Dyntick with callbacks");
                per_cpu(rcu_dyntick_drain, cpu) = 0;
-               per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
+               per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
                if (rcu_cpu_has_nonlazy_callbacks(cpu))
                        hrtimer_start(&per_cpu(rcu_idle_gp_timer, cpu),
                                      rcu_idle_gp_wait, HRTIMER_MODE_REL);