nohz: Reduce overhead under high-freq idling patterns

One testbox of mine (Intel Nehalem, 16-way) uses MWAIT for its idle routine,
which apparently can break out of its idle loop rather frequently, with
high frequency.

In that case NO_HZ_FULL=y kernels show high ksoftirqd overhead and constant
context switching, because tick_nohz_stop_sched_tick() will, if
delta_jiffies == 0, mis-identify this as a timer event - activating the
TIMER_SOFTIRQ, which wakes up ksoftirqd.

Fix this by treating delta_jiffies == 0 the same way we treat other short
wakeups, delta_jiffies == 1.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 4 insertions, 3 deletions

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 85e05ab98253..da53c8f2beb5 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -565,11 +565,12 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
                         delta_jiffies = rcu_delta_jiffies;
                 }
         }
+
         /*
-         * Do not stop the tick, if we are only one off
-         * or if the cpu is required for rcu
+         * Do not stop the tick, if we are only one off (or less)
+         * or if the cpu is required for RCU:
          */
-        if (!ts->tick_stopped && delta_jiffies == 1)
+        if (!ts->tick_stopped && delta_jiffies <= 1)
                 goto out;
 
         /* Schedule the tick, if we are at least one jiffie off */