Re: [RFC -v2 PATCH 2/3] sched: add yield_to function

Mike Galbraith
2010-12-28T05:55:06Z

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@gmx.de>
> ---
> 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 = &current->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,
>
>
>