From 2548d546d40c0014efdde88a53bf7896e917dcce Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 21 Apr 2016 18:03:15 +0200 Subject: nohz/full, sched/rt: Fix missed tick-reenabling bug in sched_can_stop_tick() Chris Metcalf reported a that sched_can_stop_tick() sometimes fails to re-enable the tick. His observed problem is that rq->cfs.nr_running can be 1 even though there are multiple runnable CFS tasks. This happens in the cgroup case, in which case cfs.nr_running is the number of runnable entities for that level. If there is a single runnable cgroup (which can have an arbitrary number of runnable child entries itself) rq->cfs.nr_running will be 1. However, looking at that function I think there's more problems with it. It seems to assume that if there's FIFO tasks, those will run. This is incorrect. The FIFO task can have a lower prio than an RR task, in which case the RR task will run. So the whole fifo_nr_running test seems misplaced, it should go after the rr_nr_running tests. That is, only if !rr_nr_running, can we use fifo_nr_running like this. Reported-by: Chris Metcalf Tested-by: Chris Metcalf Signed-off-by: Peter Zijlstra (Intel) Cc: Alexander Shishkin Cc: Arnaldo Carvalho de Melo Cc: Christoph Lameter Cc: Frederic Weisbecker Cc: Jiri Olsa Cc: Luiz Capitulino Cc: Mike Galbraith Cc: Peter Zijlstra Cc: Rik van Riel Cc: Stephane Eranian Cc: Thomas Gleixner Cc: Vince Weaver Cc: Viresh Kumar Cc: Wanpeng Li Fixes: 76d92ac305f2 ("sched: Migrate sched to use new tick dependency mask model") Link: http://lkml.kernel.org/r/20160421160315.GK24771@twins.programming.kicks-ass.net Signed-off-by: Ingo Molnar --- kernel/sched/core.c | 29 ++++++++++++++++------------- 1 file changed, 16 insertions(+), 13 deletions(-) (limited to 'kernel') diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 8b489fcac37b..d1f7149f8704 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -596,17 +596,8 @@ bool sched_can_stop_tick(struct rq *rq) return false; /* - * FIFO realtime policy runs the highest priority task (after DEADLINE). - * Other runnable tasks are of a lower priority. The scheduler tick - * isn't needed. - */ - fifo_nr_running = rq->rt.rt_nr_running - rq->rt.rr_nr_running; - if (fifo_nr_running) - return true; - - /* - * Round-robin realtime tasks time slice with other tasks at the same - * realtime priority. + * If there are more than one RR tasks, we need the tick to effect the + * actual RR behaviour. */ if (rq->rt.rr_nr_running) { if (rq->rt.rr_nr_running == 1) @@ -615,8 +606,20 @@ bool sched_can_stop_tick(struct rq *rq) return false; } - /* Normal multitasking need periodic preemption checks */ - if (rq->cfs.nr_running > 1) + /* + * If there's no RR tasks, but FIFO tasks, we can skip the tick, no + * forced preemption between FIFO tasks. + */ + fifo_nr_running = rq->rt.rt_nr_running - rq->rt.rr_nr_running; + if (fifo_nr_running) + return true; + + /* + * If there are no DL,RR/FIFO tasks, there must only be CFS tasks left; + * if there's more than one we need the tick for involuntary + * preemption. + */ + if (rq->nr_running > 1) return false; return true; -- cgit v1.2.3