Re: [RFC PATCH 3/3] sched: introduce synchronized idle injection

[Jacob Pan <jacob.jun.pan@linux.intel.com>]

On Tue, 3 Nov 2015 14:31:20 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> > @@ -5136,6 +5148,16 @@ pick_next_task_fair(struct rq *rq, struct
> > task_struct *prev) struct task_struct *p;
> >  	int new_tasks;
> >  
> > +#ifdef CONFIG_CFS_IDLE_INJECT
> > +	if (cfs_rq->force_throttled &&
> > +		!idle_cpu(cpu_of(rq)) &&
> > +		!unlikely(local_softirq_pending())) {
> > +		/* forced idle, pick no task */
> > +		trace_sched_cfs_idle_inject(cpu_of(rq), 1);
> > +		update_curr(cfs_rq);
> > +		return NULL;
> > +	}
> > +#endif
> >  again:
> >  #ifdef CONFIG_FAIR_GROUP_SCHED
> >  	if (!cfs_rq->nr_running)
> 
> So this is horrible...
> 
> This is a fast path, and you just put at least one cachemiss in it, a
> branch (without hint) and some goofy code (wth are we checking
> softirqs?).
> 
softirq is checked here since it is one of the conditions to stop
sched tick. can_stop_idle_tick(). but we don't have to check here, you
are right.
> How about you frob things such that cfs_rq->nr_running == 0 and we'll
> hit the idle: path, at that point you can test if we're forced idle
> and skip the load-balancing attempt.
> 
> There's probably a fair number of icky cases to deal with if you frob
> cfs_rq->nr_running, like the enqueue path which will add to it. We'll
> have to come up with something to not slow that down either.
> 
> The thing is, both schedule and enqueue are very hot and this is code
> that will 'never' run.
Fair enough, I will give that a try. I understand we don't want to
sacrifice the hot patch for some code almost 'never' run. But I also
have follow up plan to use this code for consolidating/synchronizing
idle during balanced semi-active workload. In that case, it may run
more often. e.g.

Before:
CPU0 ______||| ||  |___________| || || |_____
CPU1 _________||| ||  |_______| || |_______

After:

CPU0 ______||| ||  |___________| || || |_____
CPU1 ______||| ||  |___________| || |_______

The goal is to have overlapping idle time if the load is already
balanced. The energy saving can be significant.

Jacob