netdev - Re: [PATCH net-next 2/2] net: exit busy loop when another process is runnable

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Date:	Wed, 13 Apr 2016 16:51:36 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Jason Wang <jasowang@...hat.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH net-next 2/2] net: exit busy loop when another process is
 runnable

On Wed, Apr 13, 2016 at 03:28:03PM +0200, Peter Zijlstra wrote:
> On Mon, Apr 11, 2016 at 07:31:57PM +0300, Michael S. Tsirkin wrote:
> > +static bool expected_to_run_fair(struct cfs_rq *cfs_rq, s64 t)
> > +{
> > +       struct sched_entity *left;
> > +       struct sched_entity *curr = cfs_rq->curr;
> > +
> > +       if (!curr || !curr->on_rq)
> > +               return false;
> > +
> > +       left = __pick_first_entity(cfs_rq);
> > +       if (!left)
> > +               return true;
> > +
> > +       return (s64)(curr->vruntime + calc_delta_fair(t, curr) -
> > +                    left->vruntime) < 0;
> > +}
> > 
> > The reason it seems easier is because that way we can reuse
> > calc_delta_fair and don't have to do the reverse translation
> > from vruntime to nsec.
> > 
> > And I guess if we do this with interrupts disabled, and only poke
> > at the current CPU's rq, we know first entity
> > won't go away so we don't need locks? 
> 
> Nope, not true. Current isn't actually in the tree, and any other task
> is subject to being moved at any time.
> Even if current was in the tree, there is no guarantee it is
> ->rb_leftmost; imagine a task being migrated in that has a smaller
> vruntime.
> 
> So this really cannot work without locks :/
> 
> I've not thought about the actual problem you're trying to solve; but I
> figured I'd let you know this before you continue down this path.

Hmm. This is in fact called in the context of a given task,
so maybe I should just change the API and use
	&task->se
instead of 
	cfs_rq->current

:

static bool expected_to_run_fair(struct cfs_rq *cfs_rq, struct task_struct *task, s64 t)
{
       struct sched_entity *left;
       struct sched_entity *curr = &task->se;
       if (!curr || !curr->on_rq)
              return false;

       left = __pick_first_entity(cfs_rq);
       if (!left)
               return true;

       return (s64)(curr->vruntime + calc_delta_fair(t, curr) -
                    left->vruntime) < 0;
}

This way it is caller's respinsibility to make sure task
is not going away.
Left here is on tree so it's not going away, right?

-- 
MST