sched: simplify move_tasks()

The move_tasks() function is currently multiplexed with two distinct
capabilities:

1. attempt to move a specified amount of weighted load from one run
queue to another; and
2. attempt to move a specified number of tasks from one run queue to
another.

The first of these capabilities is used in two places, load_balance()
and load_balance_idle(), and in both of these cases the return value of
move_tasks() is used purely to decide if tasks/load were moved and no
notice of the actual number of tasks moved is taken.

The second capability is used in exactly one place,
active_load_balance(), to attempt to move exactly one task and, as
before, the return value is only used as an indicator of success or failure.

This multiplexing of sched_task() was introduced, by me, as part of the
smpnice patches and was motivated by the fact that the alternative, one
function to move specified load and one to move a single task, would
have led to two functions of roughly the same complexity as the old
move_tasks() (or the new balance_tasks()).  However, the new modular
design of the new CFS scheduler allows a simpler solution to be adopted
and this patch addresses that solution by:

1. adding a new function, move_one_task(), to be used by
active_load_balance(); and
2. making move_tasks() a single purpose function that tries to move a
specified weighted load and returns 1 for success and 0 for failure.

One of the consequences of these changes is that neither move_one_task()
or the new move_tasks() care how many tasks sched_class.load_balance()
moves and this enables its interface to be simplified by returning the
amount of load moved as its result and removing the load_moved pointer
from the argument list.  This helps simplify the new move_tasks() and
slightly reduces the amount of work done in each of
sched_class.load_balance()'s implementations.

Further simplification, e.g. changes to balance_tasks(), are possible
but (slightly) complicated by the special needs of load_balance_fair()
so I've left them to a later patch (if this one gets accepted).

NB Since move_tasks() gets called with two run queue locks held even
small reductions in overhead are worthwhile.

[ mingo@elte.hu ]

this change also reduces code size nicely:

   text    data     bss     dec     hex filename
   39216    3618      24   42858    a76a sched.o.before
   39173    3618      24   42815    a73f sched.o.after

Signed-off-by: Peter Williams <pwil3058@bigpond.net.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 5 insertions, 4 deletions

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 002fcf8d3f64..2b0626a43cb8 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -172,15 +172,16 @@ static struct task_struct *load_balance_next_rt(void *arg)
         return p;
 }
 
-static int
+static unsigned long
 load_balance_rt(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 cpu_idle_type idle,
-                        int *all_pinned, unsigned long *load_moved)
+                        int *all_pinned)
 {
         int this_best_prio, best_prio, best_prio_seen = 0;
         int nr_moved;
         struct rq_iterator rt_rq_iterator;
+        unsigned long load_moved;
 
         best_prio = sched_find_first_bit(busiest->rt.active.bitmap);
         this_best_prio = sched_find_first_bit(this_rq->rt.active.bitmap);
@@ -203,11 +204,11 @@ load_balance_rt(struct rq *this_rq, int this_cpu, struct rq *busiest,
         rt_rq_iterator.arg = busiest;
 
         nr_moved = balance_tasks(this_rq, this_cpu, busiest, max_nr_move,
-                        max_load_move, sd, idle, all_pinned, load_moved,
+                        max_load_move, sd, idle, all_pinned, &load_moved,
                         this_best_prio, best_prio, best_prio_seen,
                         &rt_rq_iterator);
 
-        return nr_moved;
+        return load_moved;
 }
 
 static void task_tick_rt(struct rq *rq, struct task_struct *p)