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, 46 insertions, 36 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 4680f52974e3..42029634ef5a 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2231,32 +2231,49 @@ out:
 }
 
 /*
- * move_tasks tries to move up to max_nr_move tasks and max_load_move weighted
- * load from busiest to this_rq, as part of a balancing operation within
- * "domain". Returns the number of tasks moved.
+ * move_tasks tries to move up to max_load_move weighted load from busiest to
+ * this_rq, as part of a balancing operation within domain "sd".
+ * Returns 1 if successful and 0 otherwise.
  *
  * Called with both runqueues locked.
  */
 static int move_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
-                      unsigned long max_nr_move, unsigned long max_load_move,
+                      unsigned long max_load_move,
                       struct sched_domain *sd, enum cpu_idle_type idle,
                       int *all_pinned)
 {
         struct sched_class *class = sched_class_highest;
-        unsigned long load_moved, total_nr_moved = 0, nr_moved;
-        long rem_load_move = max_load_move;
+        unsigned long total_load_moved = 0;
 
         do {
-                nr_moved = class->load_balance(this_rq, this_cpu, busiest,
-                                max_nr_move, (unsigned long)rem_load_move,
-                                sd, idle, all_pinned, &load_moved);
-                total_nr_moved += nr_moved;
-                max_nr_move -= nr_moved;
-                rem_load_move -= load_moved;
+                total_load_moved +=
+                        class->load_balance(this_rq, this_cpu, busiest,
+                                ULONG_MAX, max_load_move - total_load_moved,
+                                sd, idle, all_pinned);
                 class = class->next;
-        } while (class && max_nr_move && rem_load_move > 0);
+        } while (class && max_load_move > total_load_moved);
 
-        return total_nr_moved;
+        return total_load_moved > 0;
+}
+
+/*
+ * move_one_task tries to move exactly one task from busiest to this_rq, as
+ * part of active balancing operations within "domain".
+ * Returns 1 if successful and 0 otherwise.
+ *
+ * Called with both runqueues locked.
+ */
+static int move_one_task(struct rq *this_rq, int this_cpu, struct rq *busiest,
+                         struct sched_domain *sd, enum cpu_idle_type idle)
+{
+        struct sched_class *class;
+
+        for (class = sched_class_highest; class; class = class->next)
+                if (class->load_balance(this_rq, this_cpu, busiest,
+                                        1, ULONG_MAX, sd, idle, NULL))
+                        return 1;
+
+        return 0;
 }
 
 /*
@@ -2588,11 +2605,6 @@ find_busiest_queue(struct sched_group *group, enum cpu_idle_type idle,
  */
 #define MAX_PINNED_INTERVAL     512
 
-static inline unsigned long minus_1_or_zero(unsigned long n)
-{
-        return n > 0 ? n - 1 : 0;
-}
-
 /*
  * Check this_cpu to ensure it is balanced within domain. Attempt to move
  * tasks if there is an imbalance.
@@ -2601,7 +2613,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
                         struct sched_domain *sd, enum cpu_idle_type idle,
                         int *balance)
 {
-        int nr_moved, all_pinned = 0, active_balance = 0, sd_idle = 0;
+        int ld_moved, all_pinned = 0, active_balance = 0, sd_idle = 0;
         struct sched_group *group;
         unsigned long imbalance;
         struct rq *busiest;
@@ -2642,18 +2654,17 @@ redo:
 
         schedstat_add(sd, lb_imbalance[idle], imbalance);
 
-        nr_moved = 0;
+        ld_moved = 0;
         if (busiest->nr_running > 1) {
                 /*
                  * Attempt to move tasks. If find_busiest_group has found
                  * an imbalance but busiest->nr_running <= 1, the group is
-                 * still unbalanced. nr_moved simply stays zero, so it is
+                 * still unbalanced. ld_moved simply stays zero, so it is
                  * correctly treated as an imbalance.
                  */
                 local_irq_save(flags);
                 double_rq_lock(this_rq, busiest);
-                nr_moved = move_tasks(this_rq, this_cpu, busiest,
-                                      minus_1_or_zero(busiest->nr_running),
+                ld_moved = move_tasks(this_rq, this_cpu, busiest,
                                       imbalance, sd, idle, &all_pinned);
                 double_rq_unlock(this_rq, busiest);
                 local_irq_restore(flags);
@@ -2661,7 +2672,7 @@ redo:
                 /*
                  * some other cpu did the load balance for us.
                  */
-                if (nr_moved && this_cpu != smp_processor_id())
+                if (ld_moved && this_cpu != smp_processor_id())
                         resched_cpu(this_cpu);
 
                 /* All tasks on this runqueue were pinned by CPU affinity */
@@ -2673,7 +2684,7 @@ redo:
                 }
         }
 
-        if (!nr_moved) {
+        if (!ld_moved) {
                 schedstat_inc(sd, lb_failed[idle]);
                 sd->nr_balance_failed++;
 
@@ -2722,10 +2733,10 @@ redo:
                         sd->balance_interval *= 2;
         }
 
-        if (!nr_moved && !sd_idle && sd->flags & SD_SHARE_CPUPOWER &&
+        if (!ld_moved && !sd_idle && sd->flags & SD_SHARE_CPUPOWER &&
             !test_sd_parent(sd, SD_POWERSAVINGS_BALANCE))
                 return -1;
-        return nr_moved;
+        return ld_moved;
 
 out_balanced:
         schedstat_inc(sd, lb_balanced[idle]);
@@ -2757,7 +2768,7 @@ load_balance_newidle(int this_cpu, struct rq *this_rq, struct sched_domain *sd)
         struct sched_group *group;
         struct rq *busiest = NULL;
         unsigned long imbalance;
-        int nr_moved = 0;
+        int ld_moved = 0;
         int sd_idle = 0;
         int all_pinned = 0;
         cpumask_t cpus = CPU_MASK_ALL;
@@ -2792,12 +2803,11 @@ redo:
 
         schedstat_add(sd, lb_imbalance[CPU_NEWLY_IDLE], imbalance);
 
-        nr_moved = 0;
+        ld_moved = 0;
         if (busiest->nr_running > 1) {
                 /* Attempt to move tasks */
                 double_lock_balance(this_rq, busiest);
-                nr_moved = move_tasks(this_rq, this_cpu, busiest,
-                                        minus_1_or_zero(busiest->nr_running),
+                ld_moved = move_tasks(this_rq, this_cpu, busiest,
                                         imbalance, sd, CPU_NEWLY_IDLE,
                                         &all_pinned);
                 spin_unlock(&busiest->lock);
@@ -2809,7 +2819,7 @@ redo:
                 }
         }
 
-        if (!nr_moved) {
+        if (!ld_moved) {
                 schedstat_inc(sd, lb_failed[CPU_NEWLY_IDLE]);
                 if (!sd_idle && sd->flags & SD_SHARE_CPUPOWER &&
                     !test_sd_parent(sd, SD_POWERSAVINGS_BALANCE))
@@ -2817,7 +2827,7 @@ redo:
         } else
                 sd->nr_balance_failed = 0;
 
-        return nr_moved;
+        return ld_moved;
 
 out_balanced:
         schedstat_inc(sd, lb_balanced[CPU_NEWLY_IDLE]);
@@ -2905,8 +2915,8 @@ static void active_load_balance(struct rq *busiest_rq, int busiest_cpu)
         if (likely(sd)) {
                 schedstat_inc(sd, alb_cnt);
 
-                if (move_tasks(target_rq, target_cpu, busiest_rq, 1,
-                               ULONG_MAX, sd, CPU_IDLE, NULL))
+                if (move_one_task(target_rq, target_cpu, busiest_rq,
+                                  sd, CPU_IDLE))
                         schedstat_inc(sd, alb_pushed);
                 else
                         schedstat_inc(sd, alb_failed);