Re: [tip:sched/core] sched: Add yield_to(task, preempt) functionality

[Rik van Riel]

On 02/25/2011 07:43 PM, Venkatesh Pallipadi wrote:
> On Thu, Feb 3, 2011 at 6:12 AM, tip-bot for Mike Galbraith
> <efault@xxxxxx>  wrote:
> > Commit-ID:  d95f412200652694e63e64bfd49f0ae274a54479
> > Gitweb:     http://git.kernel.org/tip/d95f412200652694e63e64bfd49f0ae274a54479
> > Author:     Mike Galbraith<efault@xxxxxx>
> > AuthorDate: Tue, 1 Feb 2011 09:50:51 -0500
> > Committer:  Ingo Molnar<mingo@xxxxxxx>
> > CommitDate: Thu, 3 Feb 2011 14:20:33 +0100
> >
> > sched: Add yield_to(task, preempt) functionality
>
> I was looking at this patch, while trying to figure out how best to
> use next buddy to solve another unrelated to this cgroup context
> switching problem. While going through this change I see something
> that I don't really understand (inlined below). Not sure whether what
> I am looking at is a bug or I am missing something very obvious.
>
> Thanks in advance for clarification.
>
> > Currently only implemented for fair class tasks.
> >
> > Add a yield_to_task method() to the fair scheduling class. allowing the
> > caller of yield_to() to accelerate another thread in it's thread group,
> > task group.
>
> <snip>
>
> >   static void calc_load_account_idle(struct rq *this_rq);
> > @@ -5448,6 +5481,58 @@ 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.
> > + *
> > + * Returns true if we indeed boosted the target task.
> > + */
> > +bool __sched yield_to(struct task_struct *p, bool preempt)
> > +{
> > +       struct task_struct *curr = current;
> > +       struct rq *rq, *p_rq;
> > +       unsigned long flags;
> > +       bool yielded = 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 (!curr->sched_class->yield_to_task)
> > +               goto out;
> > +
> > +       if (curr->sched_class != p->sched_class)
> > +               goto out;
> > +
> > +       if (task_running(p_rq, p) || p->state)
> > +               goto out;
> > +
> > +       yielded = curr->sched_class->yield_to_task(rq, p, preempt);
> > +       if (yielded)
> > +               schedstat_inc(rq, yld_count);
> > +
> > +out:
> > +       double_rq_unlock(rq, p_rq);
> > +       local_irq_restore(flags);
> > +
> > +       if (yielded)
> > +               schedule();
> > +
> > +       return yielded;
> > +}
> > +EXPORT_SYMBOL_GPL(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.
> > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
> > index c0fbeb9..0270246 100644
> > --- a/kernel/sched_fair.c
> > +++ b/kernel/sched_fair.c
> > @@ -1975,6 +1975,25 @@ static void yield_task_fair(struct rq *rq)
> >         set_skip_buddy(se);
> >   }
> >
> > +static bool yield_to_task_fair(struct rq *rq, struct task_struct *p, bool preempt)
> > +{
> > +       struct sched_entity *se =&p->se;
> > +
> > +       if (!se->on_rq)
> > +               return false;
> > +
> > +       /* Tell the scheduler that we'd really like pse to run next. */
> > +       set_next_buddy(se);
>
> The below comment says about rescheduling p's CPU. But the rq variable
> we have here is the curr_rq and not p_rq. So, should this be done in
> yield_to() with p_rq. I did try to see the discussion on other
> versions of this patch. v3 and before had -
> "resched_task(task_of(p_cfs_rq->curr));" which seems to be correct...

You are correct.  We are calling resched_task on the wrong task,
we should call it on p's runqueue's current task...

> > +
> > +       /* Make p's CPU reschedule; pick_next_entity takes care of fairness. */
> > +       if (preempt)
> > +               resched_task(rq->curr);



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