Re: [PATCH v4] sched/core: Preempt current task in favour of bound kthread

Peter Zijlstra
2019-12-12T10:23:42Z

On Thu, Dec 12, 2019 at 11:14:24AM +0100, Peter Zijlstra wrote:
> On Thu, Dec 12, 2019 at 11:10:31AM +0100, Peter Zijlstra wrote:
> > @@ -4156,13 +4159,13 @@ check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> > if (delta_exec < sysctl_sched_min_granularity)
> > return;
> >
> > - se = __pick_first_entity(cfs_rq);
> > + se = __pick_next_entity(cfs_rq, NULL);
> > delta = curr->vruntime - se->vruntime;
> >
> > if (delta < 0)
> > return;
>
> What I mean with the below comment is, when this isn't enough, try
> something like:
>
> if (se == cfs_rq->next)
> ideal_runtime /= 2;
>
> to make it yield sooner to 'next' buddies. Sadly, due to the whole
> cgroup mess, we can't say what actual task is on the end of it (without
> doing a full hierarchy pick, which is more expensive still).

Just for giggles, that'd look something like:

while (!entity_is_task(se) {
cfs_rq = group_cfs_rq(se);
se = pick_next_entity(cfs_rq, cfs_rq->curr);
}
p = task_of(se);

if (is_per_cpu_kthread(p))
ideal_runtime /= 2;

the core-scheduling patch set includes the right primitive for this I
think, pick_task_fair().

> > - if (delta > ideal_runtime)
> > + if (delta > ideal_runtime) // maybe frob this too ?
> > resched_curr(rq_of(cfs_rq));
> > }