From 47aa8b6cbcb839efe2edaa5b50fee21df115d37b Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Fri, 26 Apr 2013 10:05:59 +0200 Subject: [PATCH] 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 Cc: Chris Metcalf Cc: Christoph Lameter Cc: Geoff Levand Cc: Gilad Ben Yossef Cc: Hakan Akkan Cc: Kevin Hilman Cc: Li Zhong Cc: Oleg Nesterov Cc: Paul E. McKenney Cc: Paul Gortmaker Cc: Peter Zijlstra Cc: Steven Rostedt Signed-off-by: Ingo Molnar --- kernel/time/tick-sched.c | 7 ++++--- 1 file 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 */ -- 2.11.0