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.
Signed-off-by: Peter Williams <[email protected]>
--
Peter Williams [email protected]
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
diff -r b97e7dab8f7b include/linux/sched.h
--- a/include/linux/sched.h Thu Aug 02 14:08:53 2007 -0700
+++ b/include/linux/sched.h Fri Aug 03 15:56:41 2007 +1000
@@ -866,11 +866,11 @@ struct sched_class {
struct task_struct * (*pick_next_task) (struct rq *rq, u64 now);
void (*put_prev_task) (struct rq *rq, struct task_struct *p, u64 now);
- int (*load_balance) (struct rq *this_rq, int this_cpu,
+ unsigned long (*load_balance) (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 *total_load_moved);
+ int *all_pinned);
void (*set_curr_task) (struct rq *rq);
void (*task_tick) (struct rq *rq, struct task_struct *p);
diff -r b97e7dab8f7b kernel/sched.c
--- a/kernel/sched.c Thu Aug 02 14:08:53 2007 -0700
+++ b/kernel/sched.c Sat Aug 04 10:06:42 2007 +1000
@@ -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);
-
- return total_nr_moved;
+ } while (class && max_load_move > total_load_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 *g
*/
#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, st
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, struc
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 r
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);
diff -r b97e7dab8f7b kernel/sched_fair.c
--- a/kernel/sched_fair.c Thu Aug 02 14:08:53 2007 -0700
+++ b/kernel/sched_fair.c Fri Aug 03 16:28:59 2007 +1000
@@ -944,11 +944,11 @@ static int cfs_rq_best_prio(struct cfs_r
return p->prio;
}
-static int
+static unsigned long
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 cpu_idle_type idle,
- int *all_pinned, unsigned long *total_load_moved)
+ int *all_pinned)
{
struct cfs_rq *busy_cfs_rq;
unsigned long load_moved, total_nr_moved = 0, nr_moved;
@@ -1006,9 +1006,7 @@ load_balance_fair(struct rq *this_rq, in
break;
}
- *total_load_moved = max_load_move - rem_load_move;
-
- return total_nr_moved;
+ return max_load_move - rem_load_move;
}
/*
diff -r b97e7dab8f7b kernel/sched_idletask.c
--- a/kernel/sched_idletask.c Thu Aug 02 14:08:53 2007 -0700
+++ b/kernel/sched_idletask.c Fri Aug 03 16:31:09 2007 +1000
@@ -37,11 +37,11 @@ static void put_prev_task_idle(struct rq
{
}
-static int
+static unsigned long
load_balance_idle(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 *total_load_moved)
+ int *all_pinned)
{
return 0;
}
diff -r b97e7dab8f7b kernel/sched_rt.c
--- a/kernel/sched_rt.c Thu Aug 02 14:08:53 2007 -0700
+++ b/kernel/sched_rt.c Fri Aug 03 16:25:14 2007 +1000
@@ -172,15 +172,16 @@ static struct task_struct *load_balance_
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
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)
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