Re: [RFC PATCH v4 00/19] Core scheduling v4

Li, Aubrey
2020-03-03T23:54:46Z

On 2020/3/3 22:59, Li, Aubrey wrote:
> On 2020/2/29 7:55, Tim Chen wrote:
>> On 2/26/20 1:54 PM, Vineeth Remanan Pillai wrote:
>>
>>> rq->curr being NULL can mean that the sibling is idle or forced idle.
>>> In both the cases, I think it makes sense to migrate a task so that it can
>>> compete with the other sibling for a chance to run. This function
>>> can_migrate_task actually only says if this task is eligible and
>>> later part of the code decides whether it is okay to migrate it
>>> based on factors like load and util and capacity. So I think its
>>> fine to declare the task as eligible if the dest core is running
>>> idle. Does this thinking make sense?
>>>
>>> On our testing, it did not show much degradation in performance with
>>> this change. I am reworking the fix by removing the check for
>>> task_est_util. It doesn't seem to be valid to check for util to migrate
>>> the task.
>>>
>>
>> In Aaron's test case, there is a great imbalance in the load on one core
>> where all the grp A tasks are vs the other cores where the grp B tasks are
>> spread around. Normally, load balancer will move the tasks for grp A.
>>
>> Aubrey's can_migrate_task patch prevented the load balancer to migrate tasks if the core
>> cookie on the target queue don't match. The thought was it will induce
>> force idle and reduces cpu utilization if we migrate task to it.
>> That kept all the grp A tasks from getting migrated and kept the imbalance
>> indefinitely in Aaron's test case.
>>
>> Perhaps we should also look at the load imbalance between the src rq and
>> target rq. If the imbalance is big (say two full cpu bound tasks worth
>> of load), we should migrate anyway despite the cookie mismatch. We are willing
>> to pay a bit for the force idle by balancing the load out more.
>> I think Aubrey's patch on can_migrate_task should be more friendly to
>> Aaron's test scenario if such logic is incorporated.
>>
>> In Vinnet's fix, we only look at the currently running task's weight in
>> src and dst rq. Perhaps the load on the src and dst rq needs to be considered
>> to prevent too great an imbalance between the run queues?
>
> We are trying to migrate a task, can we just use cfs.h_nr_running? This signal
> is used to find the busiest run queue as well.

How about this one? the cgroup weight issue seems fixed on my side.

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f42ceec..90024cf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1767,6 +1767,8 @@ static void task_numa_compare(struct task_numa_env *env,
rcu_read_unlock();
}

+static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p);
+
static void task_numa_find_cpu(struct task_numa_env *env,
long taskimp, long groupimp)
{
@@ -5650,6 +5652,44 @@ static struct sched_group *
find_idlest_group(struct sched_domain *sd, struct task_struct *p,
int this_cpu, int sd_flag);

+#ifdef CONFIG_SCHED_CORE
+static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
+{
+ struct rq *src_rq = task_rq(p);
+ bool idle_core = true;
+ int cpu;
+
+ /* Ignore cookie match if core scheduler is not enabled on the CPU. */
+ if (!sched_core_enabled(rq))
+ return true;
+
+ if (rq->core->core_cookie == p->core_cookie)
+ return true;
+
+ for_each_cpu(cpu, cpu_smt_mask(cpu_of(rq))) {
+ if (!available_idle_cpu(cpu)) {
+ idle_core = false;
+ break;
+ }
+ }
+ /*
+ * A CPU in an idle core is always the best choice for tasks with
+ * cookies.
+ */
+ if (idle_core)
+ return true;
+
+ /*
+ * Ignore cookie match if there is a big imbalance between the src rq
+ * and dst rq.
+ */
+ if ((src_rq->cfs.h_nr_running - rq->cfs.h_nr_running) > 1)
+ return true;
+
+ return false;
+}
+#endif
+
/*
* find_idlest_group_cpu - find the idlest CPU among the CPUs in the group.
*/
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 7ae6858..8c607e9 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1061,28 +1061,6 @@ static inline raw_spinlock_t *rq_lockp(struct rq *rq)
return &rq->__lock;
}

-static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
-{
- bool idle_core = true;
- int cpu;
-
- /* Ignore cookie match if core scheduler is not enabled on the CPU. */
- if (!sched_core_enabled(rq))
- return true;
-
- for_each_cpu(cpu, cpu_smt_mask(cpu_of(rq))) {
- if (!available_idle_cpu(cpu)) {
- idle_core = false;
- break;
- }
- }
- /*
- * A CPU in an idle core is always the best choice for tasks with
- * cookies.
- */
- return idle_core || rq->core->core_cookie == p->core_cookie;
-}
-
extern void queue_core_balance(struct rq *rq);

void sched_core_add(struct rq *rq, struct task_struct *p);