Re: [PATCH linux-next] sched: Fix broken setscheduler()

From: Peter Zijlstra
Date: Thu Mar 06 2014 - 06:58:51 EST


On Wed, Mar 05, 2014 at 11:29:31PM -0500, Steven Rostedt wrote:
> 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.

So Juri had a different patch for this problem:

http://lkml.kernel.org/r/20140301191838.d15d03112b2598a671dac22c@xxxxxxxxx

> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 4600bca..b1cc871 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);
> }

Now; if I'm reading things right, normal_prio is the unboosted priority
of a task. And we should indeed keep setting that, otherwise the unboost
doesn't know where it should go.

Juri put that in __setscheduler(), but I think that's wrong because the
rt_mutex_check_prio() case in __sched_setscheduler() still needs to
update this.

> @@ -3207,6 +3208,8 @@ static void __setscheduler(struct rq *rq, struct task_struct *p,
> {
> __setscheduler_params(p, attr);
>
> + p->prio = rt_mutex_getprio(p);
> +
> if (dl_prio(p->prio))
> p->sched_class = &dl_sched_class;
> else if (rt_prio(p->prio))
>

And when we call this we're sure to not be boosted; so this is
effectively the same as Juri has:

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

Seeing how rt_mutex_getprio() and normal_prio() are the same under these
conditions.


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