From: John Hawkes <hawkes@oss.sgi.com>
A large number of processes that are pinned to a single CPU results in
every other CPU's load_balance() seeing this overloaded CPU as "busiest",
yet move_tasks() never finds a task to pull-migrate. This condition occurs
during module unload, but can also occur as a denial-of-service using
sys_sched_setaffinity(). Several hundred CPUs performing this fruitless
load_balance() will livelock on the busiest CPU's runqueue lock. A smaller
number of CPUs will livelock if the pinned task count gets high. This
simple patch remedies the more common first problem: after a move_tasks()
failure to migrate anything, the balance_interval increments. Using a
simple increment, vs. the more dramatic doubling of the balance_interval,
is conservative and yet also effective.
Signed-off-by: John Hawkes <hawkes@sgi.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
25-akpm/kernel/sched.c | 16 ++++++++++++----
1 files changed, 12 insertions(+), 4 deletions(-)
diff -puN kernel/sched.c~sched-improved-load_balance-tolerance-for-pinned-tasks kernel/sched.c
--- 25/kernel/sched.c~sched-improved-load_balance-tolerance-for-pinned-tasks 2004-10-21 14:54:28.489592416 -0700
+++ 25-akpm/kernel/sched.c 2004-10-21 14:54:28.495591504 -0700
@@ -1974,11 +1974,19 @@ static int load_balance(int this_cpu, ru
*/
sd->nr_balance_failed = sd->cache_nice_tries;
}
- } else
- sd->nr_balance_failed = 0;
- /* We were unbalanced, so reset the balancing interval */
- sd->balance_interval = sd->min_interval;
+ /*
+ * We were unbalanced, but unsuccessful in move_tasks(),
+ * so bump the balance_interval to lessen the lock contention.
+ */
+ if (sd->balance_interval < sd->max_interval)
+ sd->balance_interval++;
+ } else {
+ sd->nr_balance_failed = 0;
+
+ /* We were unbalanced, so reset the balancing interval */
+ sd->balance_interval = sd->min_interval;
+ }
return nr_moved;
_