From 2b17c545a4cdbbbadcd7f1e9684c2d7db8f085a6 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Thu, 4 Oct 2012 01:46:44 +0200 Subject: nohz: Fix one jiffy count too far in idle cputime When we stop the tick in idle, we save the current jiffies value in ts->idle_jiffies. This snapshot is substracted from the later value of jiffies when the tick is restarted and the resulting delta is accounted as idle cputime. This is how we handle the idle cputime accounting without the tick. But sometimes we need to schedule the next tick to some time in the future instead of completely stopping it. In this case, a tick may happen before we restart the periodic behaviour and from that tick we account one jiffy to idle cputime as usual but we also increment the ts->idle_jiffies snapshot by one so that when we compute the delta to account, we substract the one jiffy we just accounted. To prepare for stopping the tick outside idle, we introduced a check that prevents from fixing up that ts->idle_jiffies if we are not running the idle task. But we use idle_cpu() for that and this is a problem if we run the tick while another CPU remotely enqueues a ttwu to our runqueue: CPU 0: CPU 1: tick_sched_timer() { ttwu_queue_remote() if (idle_cpu(CPU 0)) ts->idle_jiffies++; } Here, idle_cpu() notes that &rq->wake_list is not empty and hence won't consider the CPU as idle. As a result, ts->idle_jiffies won't be incremented. But this is wrong because we actually account the current jiffy to idle cputime. And that jiffy won't get substracted from the nohz time delta. So in the end, this jiffy is accounted twice. Fix this by changing idle_cpu(smp_processor_id()) with is_idle_task(current). This way the jiffy is substracted correctly even if a ttwu operation is enqueued on the CPU. Signed-off-by: Frederic Weisbecker Cc: Peter Zijlstra Cc: # 3.5+ Link: http://lkml.kernel.org/r/1349308004-3482-1-git-send-email-fweisbec@gmail.com Signed-off-by: Ingo Molnar --- kernel/time/tick-sched.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'kernel/time') diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index f423bdd035c2..a40260885265 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -835,7 +835,7 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer) */ if (ts->tick_stopped) { touch_softlockup_watchdog(); - if (idle_cpu(cpu)) + if (is_idle_task(current)) ts->idle_jiffies++; } update_process_times(user_mode(regs)); -- cgit v1.2.3