Re: [RFC][PATCH 5/6] core changes for group fairness

[Dmitry Adamushko]

On 11/06/07, Srivatsa Vaddagiri <vatsa@xxxxxxxxxxxxxxxxxx> wrote:
> This patch introduces the core changes in CFS required to accomplish
> group fairness at higher levels. It also modifies load balance interface
> between classes a bit, so that move_tasks (which is centric to load
> balance) can be reused to balance between runqueues of various types
> (struct rq in case of SCHED_RT tasks, struct lrq in case of
> SCHED_NORMAL/BATCH tasks).

a few things that catched my eye, please see below:


> +static int balance_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
> +                     unsigned long max_nr_move, unsigned long max_load_move,
> +                     struct sched_domain *sd, enum idle_type idle,
> +                     int *all_pinned, unsigned long *load_moved,
> +                     int this_best_prio, int best_prio, int best_prio_seen,
> +                     void *iterator_arg,
> +                     struct task_struct *(*iterator_start)(void *arg),
> +                     struct task_struct *(*iterator_next)(void *arg));

IMHO, it looks a bit frightening :) maybe it would be possible to
create a structure that combines some relevant argumens .. at least,
the last 3 ones.


> -static int move_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
> +static int balance_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
>                       unsigned long max_nr_move, unsigned long max_load_move,
>                       struct sched_domain *sd, enum idle_type idle,
> -                     int *all_pinned)
> +                     int *all_pinned, unsigned long *load_moved,
> +                     int this_best_prio, int best_prio, int best_prio_seen,
> +                     void *iterator_arg,
> +                     struct task_struct *(*iterator_start)(void *arg),
> +                     struct task_struct *(*iterator_next)(void *arg))

I think, there is a possible problem here. If I'm not complete wrong,
this function (move_tasks() in the current mainline) can move more
'load' than specified by the 'max_load_move'..

as a result, e.g. in the following code :

> +static int move_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
> +                     unsigned long max_nr_move, unsigned long max_load_move,
> +                     struct sched_domain *sd, enum idle_type idle,
> +                     int *all_pinned)
> +{
> +       struct sched_class *class = sched_class_highest;
> +       unsigned long load_moved, total_nr_moved = 0, nr_moved;
> +
> +       do {
> +               nr_moved = class->load_balance(this_rq, this_cpu, busiest,
> +                                       max_nr_move, max_load_move, sd, idle,
> +                                       all_pinned, &load_moved);
> +               total_nr_moved += nr_moved;
> +               max_nr_move -= nr_moved;
> +               max_load_move -= load_moved;

can become negative.. and as it's 'unsigned' --> a huge positive number..

> +               class = class->next;
> +       } while (class && max_nr_move && max_load_move);

'(long)max_load_move > 0'      ?

the same is applicable to a few other similar cases below :

> +static int
> +load_balance_fair(struct rq *this_rq, int this_cpu, struct rq *busiest,
> +                       unsigned long max_nr_move, unsigned long max_load_move,
> +                       struct sched_domain *sd, enum idle_type idle,
> +                       int *all_pinned, unsigned long *total_load_moved)
> +{
> +       struct lrq *busy_lrq;
> +       unsigned long load_moved, total_nr_moved = 0, nr_moved, rem_load_move;
> +
> +       rem_load_move = max_load_move;
> +
> +       for_each_leaf_lrq(busiest, busy_lrq) {
> +               struct lrq *this_lrq;
> +               long imbalance;
> +               unsigned long maxload;
> +               int this_best_prio, best_prio, best_prio_seen = 0;
> +
                   ..........
> +
> +               nr_moved = balance_tasks(this_rq, this_cpu, busiest,
> +                               max_nr_move, maxload, sd, idle, all_pinned,
> +                               &load_moved, this_best_prio, best_prio,
> +                               best_prio_seen,
> +                               /* pass busy_lrq argument into
> +                                * load_balance_[start|next]_fair iterators
> +                                */
> +                               busy_lrq,
> +                               load_balance_start_fair,
> +                               load_balance_next_fair);
> +
> +               total_nr_moved += nr_moved;
> +               max_nr_move -= nr_moved;
> +               rem_load_move -= load_moved;

here


> +
> +               /* todo: break if rem_load_move is < load_per_task */
> +               if (!max_nr_move || !rem_load_move)

'(long)rem_load_move <= 0'

and I think somewhere else in the code.


> --
> Regards,
> vatsa

--
Best regards,
Dmitry Adamushko
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