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Subject[PATCH v2 10/19] 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.

Running SPECjbb2005 on a 4 node machine and comparing bops/JVM
JVMS LAST_PATCH WITH_PATCH %CHANGE
16 25226.6 25436.1 0.83
1 73326 74031 0.96

Running SPECjbb2005 on a 16 node machine and comparing bops/JVM
JVMS LAST_PATCH WITH_PATCH %CHANGE
8 108750 110355 1.475
1 183115 178401 -2.57

(numbers from v1 based on v4.17-rc5)
Testcase Time: Min Max Avg StdDev
numa01.sh Real: 414.64 819.20 556.08 147.70
numa01.sh Sys: 77.52 205.04 139.40 52.05
numa01.sh User: 37043.24 61757.88 45517.48 9290.38
numa02.sh Real: 60.80 63.32 61.63 0.88
numa02.sh Sys: 17.35 39.37 25.71 7.33
numa02.sh User: 5213.79 5374.73 5268.90 55.09
numa03.sh Real: 780.09 948.64 831.43 63.02
numa03.sh Sys: 104.96 136.92 116.31 11.34
numa03.sh User: 60465.42 73339.78 64368.03 4700.14
numa04.sh Real: 412.60 681.92 521.29 96.64
numa04.sh Sys: 210.32 314.10 251.77 37.71
numa04.sh User: 34026.38 45581.20 38534.49 4198.53
numa05.sh Real: 394.79 439.63 411.35 16.87
numa05.sh Sys: 238.32 330.09 292.31 38.32
numa05.sh User: 33456.45 34876.07 34138.62 609.45

Testcase Time: Min Max Avg StdDev %Change
numa01.sh Real: 434.84 676.90 550.53 106.24 1.008%
numa01.sh Sys: 125.98 217.34 179.41 30.35 -22.3%
numa01.sh User: 38318.48 53789.56 45864.17 6620.80 -0.75%
numa02.sh Real: 60.06 61.27 60.59 0.45 1.716%
numa02.sh Sys: 14.25 17.86 16.09 1.28 59.78%
numa02.sh User: 5190.13 5225.67 5209.24 13.19 1.145%
numa03.sh Real: 748.21 960.25 823.15 73.51 1.005%
numa03.sh Sys: 96.68 122.10 110.42 11.29 5.334%
numa03.sh User: 58222.16 72595.27 63552.22 5048.87 1.283%
numa04.sh Real: 433.08 630.55 499.30 68.15 4.404%
numa04.sh Sys: 245.22 386.75 306.09 63.32 -17.7%
numa04.sh User: 35014.68 46151.72 38530.26 3924.65 0.010%
numa05.sh Real: 394.77 410.07 401.41 5.99 2.476%
numa05.sh Sys: 212.40 301.82 256.23 35.41 14.08%
numa05.sh User: 33224.86 34201.40 33665.61 313.40 1.405%

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>
---
kernel/sched/fair.c | 17 +++++++++++++++++
kernel/sched/sched.h | 1 +
2 files changed, 18 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0d0248b..50c7727 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1478,6 +1478,16 @@ 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);
+
+ if (xchg(&rq->numa_migrate_on, 1))
+ return;
+
+ 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)
@@ -1533,6 +1543,9 @@ 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)))
@@ -1699,6 +1712,7 @@ 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;
@@ -1802,14 +1816,17 @@ 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 52ba2d6..5b15c52 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -756,6 +756,7 @@ 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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