Re: [RFC PATCH v1] sched/fair: limit load balance redo times at the same sched_domain level

[Li, Aubrey]

Hi Vincent,

Sorry for the delay, I just returned from Chinese New Year holiday.

On 2021/1/25 22:51, Vincent Guittot wrote:
> On Mon, 25 Jan 2021 at 15:00, Li, Aubrey <aubrey.li@xxxxxxxxxxxxxxx> wrote:
>>
>> On 2021/1/25 18:56, Vincent Guittot wrote:
>>> On Mon, 25 Jan 2021 at 06:50, Aubrey Li <aubrey.li@xxxxxxxxx> wrote:
>>>>
>>>> A long-tail load balance cost is observed on the newly idle path,
>>>> this is caused by a race window between the first nr_running check
>>>> of the busiest runqueue and its nr_running recheck in detach_tasks.
>>>>
>>>> Before the busiest runqueue is locked, the tasks on the busiest
>>>> runqueue could be pulled by other CPUs and nr_running of the busiest
>>>> runqueu becomes 1, this causes detach_tasks breaks with LBF_ALL_PINNED
>>>
>>> We should better detect that when trying to detach task like below
>>
>> This should be a compromise from my understanding. If we give up load balance
>> this time due to the race condition, we do reduce the load balance cost on the
>> newly idle path, but if there is an imbalance indeed at the same sched_domain
> 
> Redo path is there in case, LB has found an imbalance but it can't
> move some loads from this busiest rq to dest rq because of some cpu
> affinity. So it tries to fix the imbalance by moving load onto another
> rq of the group. In your case, the imbalance has disappeared because
> it has already been pulled by another rq so you don't have to try to
> find another imbalance. And I would even say you should not in order
> to let other level to take a chance to spread the load
> 
>> level, we have to wait the next softirq entry to handle that imbalance. This
>> means the tasks on the second busiest runqueue have to stay longer, which could
>> introduce tail latency as well. That's why I introduced a variable to control
>> the redo loops. I'll send this to the benchmark queue to see if it makes any
> 
> TBH, I don't like multiplying the number of knobs

Sure, I can take your approach, :)

>>>
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -7688,6 +7688,16 @@ static int detach_tasks(struct lb_env *env)
>>>
>>>         lockdep_assert_held(&env->src_rq->lock);
>>>
>>> +       /*
>>> +        * Another CPU has emptied this runqueue in the meantime.
>>> +        * Just return and leave the load_balance properly.
>>> +        */
>>> +       if (env->src_rq->nr_running <= 1 && !env->loop) {

May I know why !env->loop is needed here? IIUC, if detach_tasks is invoked
from LBF_NEED_BREAK, env->loop could be non-zero, but as long as src_rq's
nr_running <=1, we should return immediately with LBF_ALL_PINNED flag cleared.

How about the following change?

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04a3ce20da67..1761d33accaa 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7683,8 +7683,11 @@ static int detach_tasks(struct lb_env *env)
 		 * We don't want to steal all, otherwise we may be treated likewise,
 		 * which could at worst lead to a livelock crash.
 		 */
-		if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1)
+		if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1) {
+			/* Clear the flag as we will not test any task */
+			env->flag &= ~LBF_ALL_PINNED;
 			break;
+		}
 
 		p = list_last_entry(tasks, struct task_struct, se.group_node);
 
Thanks,
-Aubrey