Re: [RFC PATCH v2 13/17] sched: Add core wide task selection and scheduling.

[Phil Auld <pauld@redhat.com>]

On Sat, May 18, 2019 at 11:37:56PM +0800 Aubrey Li wrote:
> On Wed, Apr 24, 2019 at 12:18 AM Vineeth Remanan Pillai
> <vpillai@digitalocean.com> wrote:
> >
> > From: Peter Zijlstra (Intel) <peterz@infradead.org>
> >
> > Instead of only selecting a local task, select a task for all SMT
> > siblings for every reschedule on the core (irrespective which logical
> > CPU does the reschedule).
> >
> > NOTE: there is still potential for siblings rivalry.
> > NOTE: this is far too complicated; but thus far I've failed to
> >       simplify it further.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > ---
> >  kernel/sched/core.c  | 222 ++++++++++++++++++++++++++++++++++++++++++-
> >  kernel/sched/sched.h |   5 +-
> >  2 files changed, 224 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index e5bdc1c4d8d7..9e6e90c6f9b9 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3574,7 +3574,7 @@ static inline void schedule_debug(struct task_struct *prev)
> >   * Pick up the highest-prio task:
> >   */
> >  static inline struct task_struct *
> > -pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> > +__pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >  {
> >         const struct sched_class *class;
> >         struct task_struct *p;
> > @@ -3619,6 +3619,220 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> >         BUG();
> >  }
> >
> > +#ifdef CONFIG_SCHED_CORE
> > +
> > +static inline bool cookie_match(struct task_struct *a, struct task_struct *b)
> > +{
> > +       if (is_idle_task(a) || is_idle_task(b))
> > +               return true;
> > +
> > +       return a->core_cookie == b->core_cookie;
> > +}
> > +
> > +// XXX fairness/fwd progress conditions
> > +static struct task_struct *
> > +pick_task(struct rq *rq, const struct sched_class *class, struct task_struct *max)
> > +{
> > +       struct task_struct *class_pick, *cookie_pick;
> > +       unsigned long cookie = 0UL;
> > +
> > +       /*
> > +        * We must not rely on rq->core->core_cookie here, because we fail to reset
> > +        * rq->core->core_cookie on new picks, such that we can detect if we need
> > +        * to do single vs multi rq task selection.
> > +        */
> > +
> > +       if (max && max->core_cookie) {
> > +               WARN_ON_ONCE(rq->core->core_cookie != max->core_cookie);
> > +               cookie = max->core_cookie;
> > +       }
> > +
> > +       class_pick = class->pick_task(rq);
> > +       if (!cookie)
> > +               return class_pick;
> > +
> > +       cookie_pick = sched_core_find(rq, cookie);
> > +       if (!class_pick)
> > +               return cookie_pick;
> > +
> > +       /*
> > +        * If class > max && class > cookie, it is the highest priority task on
> > +        * the core (so far) and it must be selected, otherwise we must go with
> > +        * the cookie pick in order to satisfy the constraint.
> > +        */
> > +       if (cpu_prio_less(cookie_pick, class_pick) && core_prio_less(max, class_pick))
> > +               return class_pick;
> > +
> > +       return cookie_pick;
> > +}
> > +
> > +static struct task_struct *
> > +pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> > +{
> > +       struct task_struct *next, *max = NULL;
> > +       const struct sched_class *class;
> > +       const struct cpumask *smt_mask;
> > +       int i, j, cpu;
> > +
> > +       if (!sched_core_enabled(rq))
> > +               return __pick_next_task(rq, prev, rf);
> > +
> > +       /*
> > +        * If there were no {en,de}queues since we picked (IOW, the task
> > +        * pointers are all still valid), and we haven't scheduled the last
> > +        * pick yet, do so now.
> > +        */
> > +       if (rq->core->core_pick_seq == rq->core->core_task_seq &&
> > +           rq->core->core_pick_seq != rq->core_sched_seq) {
> > +               WRITE_ONCE(rq->core_sched_seq, rq->core->core_pick_seq);
> > +
> > +               next = rq->core_pick;
> > +               if (next != prev) {
> > +                       put_prev_task(rq, prev);
> > +                       set_next_task(rq, next);
> > +               }
> > +               return next;
> > +       }
> > +
> 
> The following patch improved my test cases.
> Welcome any comments.
> 

This is certainly better than violating the point of the core scheduler :)

If I'm understanding this right what will happen in this case is instead
of using the idle process selected by the sibling we do the core scheduling
again. This may start with a newidle_balance which might bring over something
to run that matches what we want to put on the sibling. If that works then I 
can see this helping. 

But I'd be a little concerned that we could end up thrashing. Once we do core 
scheduling again here we'd force the sibling to resched and if we got a different 
result which "helped" him pick idle we'd go around again.  

I think inherent in the concept of core scheduling (barring a perfectly aligned set 
of jobs) is some extra idle time on siblings. 



Cheers,
Phil



> Thanks,
> -Aubrey
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3e3162f..86031f4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3685,10 +3685,12 @@ pick_next_task(struct rq *rq, struct
> task_struct *prev, struct rq_flags *rf)
>         /*
>          * If there were no {en,de}queues since we picked (IOW, the task
>          * pointers are all still valid), and we haven't scheduled the last
> -        * pick yet, do so now.
> +        * pick yet, do so now. If the last pick is idle task, we abandon
> +        * last pick and try to pick up task this time.
>          */
>         if (rq->core->core_pick_seq == rq->core->core_task_seq &&
> -           rq->core->core_pick_seq != rq->core_sched_seq) {
> +           rq->core->core_pick_seq != rq->core_sched_seq &&
> +           !is_idle_task(rq->core_pick)) {
>                 WRITE_ONCE(rq->core_sched_seq, rq->core->core_pick_seq);
> 
>                 next = rq->core_pick;

--