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

[Rik van Riel]

On 12/02/2010 07:50 PM, Chris Wright wrote:

> > +void requeue_task(struct rq *rq, struct task_struct *p)
> > +{
> > +	assert_spin_locked(&rq->lock);
> > +
> > +	if (!p->se.on_rq || task_running(rq, p) || task_has_rt_policy(p))
> > +		return;
>
> already checked task_running(rq, p) || task_has_rt_policy(p) w/ rq lock
> held.

OK, I removed the duplicate checks.

> > +
> > +	dequeue_task(rq, p, 0);
> > +	enqueue_task(rq, p, 0);
>
> seems like you could condense to save an update_rq_clock() call at least,
> don't know if the info_queued, info_dequeued need to be updated

Or I can do the whole operation with the task not queued.
Not sure yet what approach I'll take...

> > +#ifdef CONFIG_SCHED_HRTICK
> > +/*
> > + * Yield the CPU, giving the remainder of our time slice to task p.
> > + * Typically used to hand CPU time to another thread inside the same
> > + * process, eg. when p holds a resource other threads are waiting for.
> > + * Giving priority to p may help get that resource released sooner.
> > + */
> > +void yield_to(struct task_struct *p)
> > +{
> > +	unsigned long flags;
> > +	struct sched_entity *se =&p->se;
> > +	struct rq *rq;
> > +	struct cfs_rq *cfs_rq;
> > +	u64 remain = slice_remain(current);
> > +
> > +	rq = task_rq_lock(p,&flags);
> > +	if (task_running(rq, p) || task_has_rt_policy(p))
> > +		goto out;
> > +	cfs_rq = cfs_rq_of(se);
> > +	se->vruntime -= remain;
> > +	if (se->vruntime<  cfs_rq->min_vruntime)
> > +		se->vruntime = cfs_rq->min_vruntime;
>
> Should these details all be in sched_fair?  Seems like the wrong layer
> here.  And would that condition go the other way?  If new vruntime is
> smaller than min, then it becomes new cfs_rq->min_vruntime?

That would be nice.  Unfortunately, EXPORT_SYMBOL() does
not seem to work right from sched_fair.c, which is included
from sched.c instead of being built from the makefile!

> > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> > index 5119b08..2a0a595 100644
> > --- a/kernel/sched_fair.c
> > +++ b/kernel/sched_fair.c
> > @@ -974,6 +974,25 @@ entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr, int queued)
> >    */
> >
> >   #ifdef CONFIG_SCHED_HRTICK
> > +u64 slice_remain(struct task_struct *p)
> > +{
> > +	unsigned long flags;
> > +	struct sched_entity *se =&p->se;
> > +	struct cfs_rq *cfs_rq;
> > +	struct rq *rq;
> > +	u64 slice, ran;
> > +	s64 delta;
> > +
> > +	rq = task_rq_lock(p,&flags);
> > +	cfs_rq = cfs_rq_of(se);
> > +	slice = sched_slice(cfs_rq, se);
> > +	ran = se->sum_exec_runtime - se->prev_sum_exec_runtime;
> > +	delta = slice - ran;
> > +	task_rq_unlock(rq,&flags);
> > +
> > +	return max(delta, 0LL);
>
> Can delta go negative?

Good question.  I don't know.

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