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sched: Guarantee task priority in pick_next_task()

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Michael spotted that the idle_balance() push down created a task
priority problem.

Previously, when we called idle_balance() before pick_next_task() it
wasn't a problem when -- because of the rq->lock droppage -- an rt/dl
task slipped in.

Similarly for pre_schedule(), rt pre-schedule could have a dl task
slip in.

But by pulling it into the pick_next_task() loop, we'll not try a
higher task priority again.

Cure this by creating a re-start condition in pick_next_task(); and
triggering this from pick_next_task_{rt,fair}().

It also fixes a live-lock where we get stuck in pick_next_task_fair()
due to idle_balance() seeing !0 nr_running but there not actually
being any fair tasks about.

Reported-by: Michael Wang <wangyun@linux.vnet.ibm.com>
Fixes: 38033c37fa ("sched: Push down pre_schedule() and idle_balance()")
Tested-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20140224121218.GR15586@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra
2014-02-14 12:25:08 +01:00
committed by Ingo Molnar
parent 06d50c65b1
commit 37e117c07b
4 changed files with 34 additions and 6 deletions
Download Patch File Download Diff File Expand all files Collapse all files

10
kernel/sched/rt.c
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@@ -1360,8 +1360,16 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev)
struct task_struct *p;
struct rt_rq *rt_rq = &rq->rt;
if (need_pull_rt_task(rq, prev))
if (need_pull_rt_task(rq, prev)) {
pull_rt_task(rq);
/*
* pull_rt_task() can drop (and re-acquire) rq->lock; this
* means a dl task can slip in, in which case we need to
* re-start task selection.
*/
if (unlikely(rq->dl.dl_nr_running))
return RETRY_TASK;
}
if (!rt_rq->rt_nr_running)
return NULL;