On Mon, 2010-12-20 at 17:04 +0100, Mike Galbraith wrote: > On Mon, 2010-12-20 at 10:40 -0500, Rik van Riel wrote: > > On 12/17/2010 02:15 AM, Mike Galbraith wrote: > > > > > BTW, with this vruntime donation thingy, what prevents a task from > > > forking off accomplices who do nothing but wait for a wakeup and > > > yield_to(exploit)? > > > > > > Even swapping vruntimes in the same cfs_rq is dangerous as hell, because > > > one party is going backward. > > > > I just realized the answer to this question. > > > > We only give cpu time to tasks that are runnable, but not > > currently running. That ensures one task cannot block others > > from running by having time yielded to it constantly. > > Hm. Don't think that will 100% sure prevent clock stoppage, because the > running task doesn't necessarily advance min_vruntime. > > What about something like the below instead? It compiles, but may eat > your first born child. (either it's christmas, or nobody cares to try it. ho ho hum:) > sched: implement fair class yield_to(task) using cfs_rq->next as a selection hint. > > <CHANGELOG> > > Not-signed-off-by: Mike Galbraith <efault@xxxxxx> > --- > include/linux/sched.h | 1 > kernel/sched.c | 47 +++++++++++++++++++++++++++++++++++++++++ > kernel/sched_fair.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++ > 3 files changed, 104 insertions(+) > > Index: linux-2.6/include/linux/sched.h > =================================================================== > --- linux-2.6.orig/include/linux/sched.h > +++ linux-2.6/include/linux/sched.h > @@ -1056,6 +1056,7 @@ struct sched_class { > void (*enqueue_task) (struct rq *rq, struct task_struct *p, int flags); > void (*dequeue_task) (struct rq *rq, struct task_struct *p, int flags); > void (*yield_task) (struct rq *rq); > + int (*yield_to_task) (struct task_struct *p, int preempt); > > void (*check_preempt_curr) (struct rq *rq, struct task_struct *p, int flags); > > Index: linux-2.6/kernel/sched.c > =================================================================== > --- linux-2.6.orig/kernel/sched.c > +++ linux-2.6/kernel/sched.c > @@ -5325,6 +5325,53 @@ void __sched yield(void) > } > EXPORT_SYMBOL(yield); > > +/** > + * yield_to - yield the current processor to another thread in > + * your thread group, or accelerate that thread toward the > + * processor it's on. > + * > + * It's the caller's job to ensure that the target task struct > + * can't go away on us before we can do any checks. > + */ > +void __sched yield_to(struct task_struct *p, int preempt) > +{ > + struct task_struct *curr = current; > + struct rq *rq, *p_rq; > + unsigned long flags; > + int yield = 0; > + > + local_irq_save(flags); > + rq = this_rq(); > + > +again: > + p_rq = task_rq(p); > + double_rq_lock(rq, p_rq); > + while (task_rq(p) != p_rq) { > + double_rq_unlock(rq, p_rq); > + goto again; > + } > + > + if (task_running(p_rq, p) || p->state || !p->se.on_rq || > + !same_thread_group(p, curr) || > + !curr->sched_class->yield_to_task || > + curr->sched_class != p->sched_class) { > + goto out; > + } > + > + yield = curr->sched_class->yield_to_task(p, preempt); > + > +out: > + double_rq_unlock(rq, p_rq); > + local_irq_restore(flags); > + > + if (yield) { > + set_current_state(TASK_RUNNING); > + schedule(); > + } > +} > +EXPORT_SYMBOL(yield_to); > + > + > /* > * This task is about to go to sleep on IO. Increment rq->nr_iowait so > * that process accounting knows that this is a task in IO wait state. > Index: linux-2.6/kernel/sched_fair.c > =================================================================== > --- linux-2.6.orig/kernel/sched_fair.c > +++ linux-2.6/kernel/sched_fair.c > @@ -1320,6 +1320,61 @@ static void yield_task_fair(struct rq *r > } > > #ifdef CONFIG_SMP > +static void pull_task(struct rq *src_rq, struct task_struct *p, > + struct rq *this_rq, int this_cpu); > +#endif > + > +static int yield_to_task_fair(struct task_struct *p, int preempt) > +{ > + struct sched_entity *se = ¤t->se; > + struct sched_entity *pse = &p->se; > + struct sched_entity *curr = &(task_rq(p)->curr)->se; > + struct cfs_rq *cfs_rq = cfs_rq_of(se); > + struct cfs_rq *p_cfs_rq = cfs_rq_of(pse); > + int yield = this_rq() == task_rq(p); > + int want_preempt = preempt; > + > +#ifdef CONFIG_FAIR_GROUP_SCHED > + if (cfs_rq->tg != p_cfs_rq->tg) > + return 0; > + > + /* Preemption only allowed within the same task group. */ > + if (preempt && cfs_rq->tg != cfs_rq_of(curr)->tg) > + preempt = 0; > +#endif > + /* Preemption only allowed within the same thread group. */ > + if (preempt && !same_thread_group(current, task_of(p_cfs_rq->curr))) > + preempt = 0; > + > +#ifdef CONFIG_SMP > + /* > + * If this yield is important enough to want to preempt instead > + * of only dropping a ->next hint, we're alone, and the target > + * is not alone, pull the target to this cpu. > + */ > + if (want_preempt && !yield && cfs_rq->nr_running == 1 && > + cpumask_test_cpu(smp_processor_id(), &p->cpus_allowed)) { > + pull_task(task_rq(p), p, this_rq(), smp_processor_id()); > + p_cfs_rq = cfs_rq_of(pse); > + yield = 1; > + } > +#endif > + > + if (yield) > + clear_buddies(cfs_rq, se); > + else if (preempt) > + clear_buddies(p_cfs_rq, curr); > + > + /* Tell the scheduler that we'd really like pse to run next. */ > + p_cfs_rq->next = pse; > + > + if (!yield && preempt) > + resched_task(task_of(p_cfs_rq->curr)); > + > + return yield; > +} > + > +#ifdef CONFIG_SMP > > static void task_waking_fair(struct rq *rq, struct task_struct *p) > { > @@ -4126,6 +4181,7 @@ static const struct sched_class fair_sch > .enqueue_task = enqueue_task_fair, > .dequeue_task = dequeue_task_fair, > .yield_task = yield_task_fair, > + .yield_to_task = yield_to_task_fair, > > .check_preempt_curr = check_preempt_wakeup, > > > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html