----- On Sep 20, 2016, at 4:49 PM, Thomas Gleixner t...@linutronix.de wrote:
> On Fri, 16 Sep 2016, Julien Desfossez wrote: > >> Tasks with RT or deadline scheduling class may inherit from a task with >> a "fair" scheduling class. > > This makes no sense. A RT/DL task can never inherit anything from a sched > fair task. That would be inverted priority inheritance. > >> This priority inheritance changes the scheduling class, but not the task >> "policy" field. >> >> Therefore, the fair scheduler should not assume that policy != >> SCHED_NORMAL is the same as (policy == SCHED_BATCH || policy == >> SCHED_IDLE), because the policy could also be SCHED_RR, SCHED_FIFO, or >> SCHED_DEADLINE. >> >> The incorrect comparison in check_preempt_wakeup makes RR, FIFO and >> DEADLINE tasks which inherit from a fair task behave as if they were >> IDLE or BATCH tasks, thus awaiting the following tick before preempting >> the current task. > > This is just wrong. > > Priority/deadline inheritance elevates a fair task to RR/FIFO/DL, i.e. to > the scheduling class of the task which is blocked on a resource held by the > fair task. > > The check_preempt_curr() callback of a scheduling class is only invoked > when the freshly woken task is in the same scheduling class as the task > which is currently on the cpu. > > So which problem are you actually solving? So what is then puzzling us is this: rt_mutex_setprio() if (dl_prio(prio)) { struct task_struct *pi_task = rt_mutex_get_top_task(p); if (!dl_prio(p->normal_prio) || (pi_task && dl_entity_preempt(&pi_task->dl, &p->dl))) { p->dl.dl_boosted = 1; queue_flag |= ENQUEUE_REPLENISH; } else p->dl.dl_boosted = 0; p->sched_class = &dl_sched_class; } else if (rt_prio(prio)) { if (dl_prio(oldprio)) p->dl.dl_boosted = 0; if (oldprio < prio) queue_flag |= ENQUEUE_HEAD; p->sched_class = &rt_sched_class; } else { if (dl_prio(oldprio)) p->dl.dl_boosted = 0; if (rt_prio(oldprio)) p->rt.timeout = 0; p->sched_class = &fair_sched_class; } So in the 3rd block, this is the case where we inherit a new prio which is neither LD nor RT, so it's "fair". If we try to assign a fair prio to a task of DL or RT prio, the dl_boosted is set to 0, or the rt timeout is set to 0. However, we do change the sched_class of the target task to &fair_sched_class. This code path seems to imply that a RT or DL task can change sched_class to "fair". Indeed, it makes no sense, so I have the feeling we're missing something important here. > >> Cc: Peter Zijlstra <pet...@infradead.org> >> Cc: Steven Rostedt (Red Hat) <rost...@goodmis.org> >> Cc: Thomas Gleixner <t...@linutronix.de> >> Cc: Ingo Molnar <mi...@redhat.com> >> Signed-off-by: Mathieu Desnoyers <mathieu.desnoy...@efficios.com> >> Signed-off-by: Julien Desfossez <jdesfos...@efficios.com> > > Who wrote the patch? Julien is the author. Thanks, Mathieu > > Thanks, > > tglx -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com