[PATCH v2] sched: Fix broken setscheduler()

From: Steven Rostedt
Date: Thu Mar 06 2014 - 12:04:55 EST


On Thu, 6 Mar 2014 13:20:40 +0100
Juri Lelli <juri.lelli@xxxxxxxxx> wrote:

> Yes. I think you can go with
>
> p->prio = p->normal_prio
>
> and save a few checks in rt_mutex_getprio().
>

You're right! Thanks.

-----

I decided to run my tests on linux-next, and my wakeup_rt tracer was
broken. After running a bisect, I found that the problem commit was:

linux-next commit c365c292d059
"sched: Consider pi boosting in setscheduler()"

And the reason the wake_rt tracer test was failing, was because it had
no RT task to trace. I first noticed this when running with
sched_switch event and saw that my RT task still had normal SCHED_OTHER
priority. Looking at the problem commit, I found:

- p->normal_prio = normal_prio(p);
- p->prio = rt_mutex_getprio(p);

With no

+ p->normal_prio = normal_prio(p);
+ p->prio = rt_mutex_getprio(p);

Reading what the commit is suppose to do, I realize that the p->prio
can't be set if the task is boosted with a higher prio, but the
p->normal_prio still needs to be set regardless, otherwise, when the
task is deboosted, it wont get the new priority.

The p->prio has to be set before "check_class_changed()" is called,
otherwise the class wont be changed.

Link: http://lkml.kernel.org/r/20140305232931.449eef8c@xxxxxxxxxxxxxxxxxx

Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
kernel/sched/core.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4600bca..3134756 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3198,6 +3198,7 @@ static void __setscheduler_params(struct task_struct *p,
* getparam()/getattr() don't report silly values for !rt tasks.
*/
p->rt_priority = attr->sched_priority;
+ p->normal_prio = normal_prio(p);
set_load_weight(p);
}

@@ -3207,6 +3208,12 @@ static void __setscheduler(struct rq *rq, struct task_struct *p,
{
__setscheduler_params(p, attr);

+ /*
+ * If we get here, there was no pi waiters boosting the
+ * task. It is safe to use the normal prio.
+ */
+ p->prio = normal_prio(p);
+
if (dl_prio(p->prio))
p->sched_class = &dl_sched_class;
else if (rt_prio(p->prio))
--
1.8.1.4

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