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SubjectRe: [RFC II] Splitting scheduler into two halves

* Mike Galbraith <umgwanakikbuti@gmail.com> wrote:

> On Thu, 2014-03-27 at 02:37 +0800, Yuyang du wrote:
> > Hi all,
> >
> > This is continued after the first RFC about splitting the scheduler. Still
> > work-in-progress, and call for feedback.
> >
> > The question addressed here is how load balance should be changed. And I think
> > the question then goes to how to *reuse* common code as much as possible and
> > meanwhile be able to serve various objectives.
> >
> > So these are the basic semantics needed in current load balance:
>
> I'll probably regret it, but I'm gonna speak my mind. I think this
> two halves concept is fundamentally broken.

As PeterZ pointed it out in the previous discussion, this approach,
besides being fundamentally broken, also gives no valid technical
rationale given for the change.

Firstly, I'd like to stress it that we are not against abstraction and
interfaces within the scheduler (at all!) - we already have a 'split'
and use interfaces between 'scheduler classes':

struct sched_class {
const struct sched_class *next;

void (*enqueue_task) (struct rq *rq, struct task_struct *p, int flags);
void (*dequeue_task) (struct rq *rq, struct task_struct *p, int flags);
void (*yield_task) (struct rq *rq);
bool (*yield_to_task) (struct rq *rq, struct task_struct *p, bool preempt);

void (*check_preempt_curr) (struct rq *rq, struct task_struct *p, int flags);

/*
* It is the responsibility of the pick_next_task() method that will
* return the next task to call put_prev_task() on the @prev task or
* something equivalent.
*
* May return RETRY_TASK when it finds a higher prio class has runnable
* tasks.
*/
struct task_struct * (*pick_next_task) (struct rq *rq,
struct task_struct *prev);
void (*put_prev_task) (struct rq *rq, struct task_struct *p);

#ifdef CONFIG_SMP
int (*select_task_rq)(struct task_struct *p, int task_cpu, int sd_flag, int flags);
void (*migrate_task_rq)(struct task_struct *p, int next_cpu);

void (*post_schedule) (struct rq *this_rq);
void (*task_waking) (struct task_struct *task);
void (*task_woken) (struct rq *this_rq, struct task_struct *task);

void (*set_cpus_allowed)(struct task_struct *p,
const struct cpumask *newmask);

void (*rq_online)(struct rq *rq);
void (*rq_offline)(struct rq *rq);
#endif

void (*set_curr_task) (struct rq *rq);
void (*task_tick) (struct rq *rq, struct task_struct *p, int queued);
void (*task_fork) (struct task_struct *p);
void (*task_dead) (struct task_struct *p);

void (*switched_from) (struct rq *this_rq, struct task_struct *task);
void (*switched_to) (struct rq *this_rq, struct task_struct *task);
void (*prio_changed) (struct rq *this_rq, struct task_struct *task,
int oldprio);

unsigned int (*get_rr_interval) (struct rq *rq,
struct task_struct *task);

#ifdef CONFIG_FAIR_GROUP_SCHED
void (*task_move_group) (struct task_struct *p, int on_rq);
#endif
};

So where it makes sense we make use of this programming technique, to
the extent it is helpful.

But interfaces and abstraction has a cost, and the justification given
in this submission looks very weak to me. There's no justification
given in this specific submission, the closest I could find was in the
first submission:

> > With the advent of more cores and heterogeneous architectures, the
> > scheduler is required to be more complex (power efficiency) and
> > diverse (big.little). For the scheduler to address that challenge
> > as a whole, it is costly but not necessary. This proposal argues
> > that the scheduler be spitted into two parts: top half (task
> > scheduling) and bottom half (load balance). Let the bottom half
> > take charge of the incoming requirements.

That is just way too generic with no specific technical benefits
listed. No cost/benefit demonstrated.

If there's any advantage to a 'split', then it must be expressable via
one or more of these positive attributes:

- better numbers (better performance, etc.)
- reduced code
- new features

A split alone, without making active and convincing use of it, is
inadequate.

So without a much better rationale, demonstrated via actual, real
working code that not only does the split but also makes real use of
every aspect of the proposed abstraction interfaces, which
demonstrates that the proposed 'split' is the most sensible way
forward, this specific submission earns a NAK from me.

Thanks,

Ingo


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