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SubjectRe: [PATCH] sched: Make sure task has correct sched_class after policy change
On 23/09/09 16:32, Peter Zijlstra wrote:
> On Wed, 2009-09-23 at 02:21 +0000, Peter Williams wrote:
>> From the code in rt_mutex_setprio(), it is evident that the intention
>> is that task's with a RT 'prio' value as a consequence of receiving a PI
>> boost also have their 'sched_class' field set to '&rt_sched_class'.
>> However, the code in __setscheduler() could result in this intention
>> being frustrated.
>>
>> This patch fixes this problem.
>>
>> Signed-off-by: Peter Williams<pwil3058@bigpond.net.au>
>
> I think you're right, but the problem seems to be that it sets
> sched_class based on policy, which seems fragile in the face of PI.
>
> How about the alternative below?

Yes, that's what I meant to do i.e. use p->prio in the if condition. I
sent a second patch with a fix but your solution is neater :-).

>
> ---
> kernel/sched.c | 17 ++++-------------
> 1 files changed, 4 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 91843ba..753a52c 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -6129,23 +6129,14 @@ __setscheduler(struct rq *rq, struct task_struct *p, int policy, int prio)
> {
> BUG_ON(p->se.on_rq);
>
> - p->policy = policy;
> - switch (p->policy) {
> - case SCHED_NORMAL:
> - case SCHED_BATCH:
> - case SCHED_IDLE:
> - p->sched_class =&fair_sched_class;
> - break;
> - case SCHED_FIFO:
> - case SCHED_RR:
> - p->sched_class =&rt_sched_class;
> - break;
> - }
> -
> p->rt_priority = prio;
> p->normal_prio = normal_prio(p);
> /* we are holding p->pi_lock already */
> p->prio = rt_mutex_getprio(p);
> + if (rt_prio(p->prio))
> + p->sched_class =&rt_sched_class;
> + else
> + p->sched_class =&fair_sched_class;
> set_load_weight(p);
> }

Peter
--
Peter Williams pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce


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