Commit-ID: 2548d546d40c0014efdde88a53bf7896e917dcce
Gitweb: http://git.kernel.org/tip/2548d546d40c0014efdde88a53bf7896e917dcce
Author: Peter Zijlstra <pet...@infradead.org>
AuthorDate: Thu, 21 Apr 2016 18:03:15 +0200
Committer: Ingo Molnar <mi...@kernel.org>
CommitDate: Thu, 28 Apr 2016 10:28:55 +0200
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 <cmetc...@mellanox.com>
Tested-by: Chris Metcalf <cmetc...@mellanox.com>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: Alexander Shishkin <alexander.shish...@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <a...@redhat.com>
Cc: Christoph Lameter <c...@linux.com>
Cc: Frederic Weisbecker <fweis...@gmail.com>
Cc: Jiri Olsa <jo...@redhat.com>
Cc: Luiz Capitulino <lcapitul...@redhat.com>
Cc: Mike Galbraith <efa...@gmx.de>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Rik van Riel <r...@redhat.com>
Cc: Stephane Eranian <eran...@google.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Vince Weaver <vincent.wea...@maine.edu>
Cc: Viresh Kumar <viresh.ku...@linaro.org>
Cc: Wanpeng Li <kernel...@gmail.com>
Fixes: 76d92ac305f2 ("sched: Migrate sched to use new tick dependency mask
model")
Link:
http://lkml.kernel.org/r/20160421160315.gk24...@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mi...@kernel.org>
---
kernel/sched/core.c | 29 ++++++++++++++++-------------
1 file changed, 16 insertions(+), 13 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8b489fc..d1f7149 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;
