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Subject[RFC PATCH 08/10] sched: Prevent a task from migrating immediately after an active balance
Date
Once a task has been carefully chosen to be migrated to a destination
node from a source node, try to avoid some other cpus moving this task
away from the destinagtion node.

If not, tasks might end up being in a ping-pong; one cpu pulling it
because of numa affinity, the other cpu pulling it for the slight
imbalance created because of previous migration (instead of actually
trying to pull a task that might lead to more consolidation)

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
include/linux/sched.h | 1 +
kernel/sched/core.c | 1 +
kernel/sched/fair.c | 9 +++++++++
3 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index a77c3cd..ba188f1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1506,6 +1506,7 @@ struct task_struct {
unsigned int numa_scan_period;
u64 node_stamp; /* migration stamp */
struct callback_head numa_work;
+ int migrate_seq;
#endif /* CONFIG_NUMA_BALANCING */

struct rcu_head rcu;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e792312..453d989 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1594,6 +1594,7 @@ static void __sched_fork(struct task_struct *p)
p->numa_migrate_seq = p->mm ? p->mm->numa_scan_seq - 1 : 0;
p->numa_scan_period = sysctl_numa_balancing_scan_delay;
p->numa_work.next = &p->numa_work;
+ p->migrate_seq = 0;
#endif /* CONFIG_NUMA_BALANCING */
}

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e04703e..a99aebc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1014,6 +1014,7 @@ static void account_numa_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p)
struct rq *rq = rq_of(cfs_rq);
int curnode = cpu_to_node(cpu_of(rq));

+ p->migrate_seq = 0;
if (mm && mm->numa_weights) {
atomic_dec(&mm->numa_weights[curnode]);
atomic_dec(&mm->numa_weights[nr_node_ids]);
@@ -4037,6 +4038,13 @@ static bool preferred_node(struct task_struct *p, struct lb_env *env)
if (!(env->sd->flags & SD_NUMA))
return false;

+ if (env->iterations) {
+ if (!p->migrate_seq)
+ return true;
+
+ p->migrate_seq--;
+ return false;
+ }
return (can_numa_migrate_task(p, env->dst_rq, env->src_rq) == 1);
}
#else
@@ -4063,6 +4071,7 @@ static int move_one_task(struct lb_env *env)
if (p->on_rq && task_cpu(p) == env->src_rq->cpu) {
move_task(p, env);
schedstat_inc(env->sd, lb_gained[env->idle]);
+ p->migrate_seq = 3;
return 1;
}
return 0;
--
1.7.1


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