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Subject[PATCH 1/6] sched/numa: Stop multiple tasks from moving to the cpu at the same time
Date
Task migration under numa balancing can happen in parallel. More than
one task might choose to migrate to the same cpu at the same time. This
can result in
- During task swap, choosing a task that was not part of the evaluation.
- During task swap, task which just got moved into its preferred node,
moving to a completely different node.
- During task swap, task failing to move to the preferred node, will have
to wait an extra interval for the next migrate opportunity.
- During task movement, multiple task movements can cause load imbalance.

This problem is more likely if there are more cores per node or more
nodes in the system.

Use a per run-queue variable to check if numa-balance is active on the
run-queue.

specjbb2005 / bops/JVM / higher bops are better
on 2 Socket/2 Node Intel
JVMS Prev Current %Change
4 199709 206350 3.32534
1 330830 319963 -3.28477


on 2 Socket/4 Node Power8 (PowerNV)
JVMS Prev Current %Change
8 89011.9 89627.8 0.69193
1 218946 211338 -3.47483


on 2 Socket/2 Node Power9 (PowerNV)
JVMS Prev Current %Change
4 180473 186539 3.36117
1 212805 220344 3.54268


on 4 Socket/4 Node Power7
JVMS Prev Current %Change
8 56941.8 56836 -0.185804
1 111686 112970 1.14965


dbench / transactions / higher numbers are better
on 2 Socket/2 Node Intel
count Min Max Avg Variance %Change
5 12029.8 12124.6 12060.9 34.0076
5 13136.1 13170.2 13150.2 14.7482 9.03166


on 2 Socket/4 Node Power8 (PowerNV)
count Min Max Avg Variance %Change
5 4968.51 5006.62 4981.31 13.4151
5 4319.79 4998.19 4836.53 261.109 -2.90646


on 2 Socket/2 Node Power9 (PowerNV)
count Min Max Avg Variance %Change
5 9342.92 9381.44 9363.92 12.8587
5 9325.56 9402.7 9362.49 25.9638 -0.0152714


on 4 Socket/4 Node Power7
count Min Max Avg Variance %Change
5 143.4 188.892 170.225 16.9929
5 132.581 191.072 170.554 21.6444 0.193274

Acked-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
Changelog v2->v3:
Add comments as requested by Peter.

kernel/sched/fair.c | 22 ++++++++++++++++++++++
kernel/sched/sched.h | 1 +
2 files changed, 23 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 309c93f..5cf921a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1514,6 +1514,21 @@ struct task_numa_env {
static void task_numa_assign(struct task_numa_env *env,
struct task_struct *p, long imp)
{
+ struct rq *rq = cpu_rq(env->dst_cpu);
+
+ /* Bail out if run-queue part of active numa balance. */
+ if (xchg(&rq->numa_migrate_on, 1))
+ return;
+
+ /*
+ * Clear previous best_cpu/rq numa-migrate flag, since task now
+ * found a better cpu to move/swap.
+ */
+ if (env->best_cpu != -1) {
+ rq = cpu_rq(env->best_cpu);
+ WRITE_ONCE(rq->numa_migrate_on, 0);
+ }
+
if (env->best_task)
put_task_struct(env->best_task);
if (p)
@@ -1569,6 +1584,9 @@ static void task_numa_compare(struct task_numa_env *env,
long moveimp = imp;
int dist = env->dist;

+ if (READ_ONCE(dst_rq->numa_migrate_on))
+ return;
+
rcu_read_lock();
cur = task_rcu_dereference(&dst_rq->curr);
if (cur && ((cur->flags & PF_EXITING) || is_idle_task(cur)))
@@ -1710,6 +1728,7 @@ static int task_numa_migrate(struct task_struct *p)
.best_cpu = -1,
};
struct sched_domain *sd;
+ struct rq *best_rq;
unsigned long taskweight, groupweight;
int nid, ret, dist;
long taskimp, groupimp;
@@ -1811,14 +1830,17 @@ static int task_numa_migrate(struct task_struct *p)
*/
p->numa_scan_period = task_scan_start(p);

+ best_rq = cpu_rq(env.best_cpu);
if (env.best_task == NULL) {
ret = migrate_task_to(p, env.best_cpu);
+ WRITE_ONCE(best_rq->numa_migrate_on, 0);
if (ret != 0)
trace_sched_stick_numa(p, env.src_cpu, env.best_cpu);
return ret;
}

ret = migrate_swap(p, env.best_task, env.best_cpu, env.src_cpu);
+ WRITE_ONCE(best_rq->numa_migrate_on, 0);

if (ret != 0)
trace_sched_stick_numa(p, env.src_cpu, task_cpu(env.best_task));
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4a2e8ca..0b91612 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -783,6 +783,7 @@ struct rq {
#ifdef CONFIG_NUMA_BALANCING
unsigned int nr_numa_running;
unsigned int nr_preferred_running;
+ unsigned int numa_migrate_on;
#endif
#define CPU_LOAD_IDX_MAX 5
unsigned long cpu_load[CPU_LOAD_IDX_MAX];
--
1.8.3.1
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