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[PATCH] sched: less locking 

During periodic load balancing, don't hold this runqueue's lock while
scanning remote runqueues, which can take a non trivial amount of time
especially on very large systems.

Holding the runqueue lock will only help to stabilise ->nr_running, however
this doesn't do much to help because tasks being woken will simply get held
up on the runqueue lock, so ->nr_running would not provide a really
accurate picture of runqueue load in that case anyway.

What's more, ->nr_running (and possibly the cpu_load averages) of remote
runqueues won't be stable anyway, so load balancing is always an inexact
operation.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Nick Piggin 2005-09-10 00:26:18 -07:00 committed by Linus Torvalds
parent d6d5cfaf45
commit e17224bf1d
1 changed files with 2 additions and 7 deletions
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9
kernel/sched.c
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@ -2075,7 +2075,6 @@ static int load_balance(int this_cpu, runqueue_t *this_rq,
int nr_moved, all_pinned = 0;
int active_balance = 0;
spin_lock(&this_rq->lock);
schedstat_inc(sd, lb_cnt[idle]);
group = find_busiest_group(sd, this_cpu, &imbalance, idle);
@ -2102,18 +2101,16 @@ static int load_balance(int this_cpu, runqueue_t *this_rq,
* still unbalanced. nr_moved simply stays zero, so it is
* correctly treated as an imbalance.
*/
double_lock_balance(this_rq, busiest);
double_rq_lock(this_rq, busiest);
nr_moved = move_tasks(this_rq, this_cpu, busiest,
imbalance, sd, idle, &all_pinned);
spin_unlock(&busiest->lock);
double_rq_unlock(this_rq, busiest);
/* All tasks on this runqueue were pinned by CPU affinity */
if (unlikely(all_pinned))
goto out_balanced;
}
spin_unlock(&this_rq->lock);
if (!nr_moved) {
schedstat_inc(sd, lb_failed[idle]);
sd->nr_balance_failed++;
@ -2156,8 +2153,6 @@ static int load_balance(int this_cpu, runqueue_t *this_rq,
return nr_moved;
out_balanced:
spin_unlock(&this_rq->lock);
schedstat_inc(sd, lb_balanced[idle]);
sd->nr_balance_failed = 0;