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

[Li, Aubrey]

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

Here is one simple case I have seen:
1) CPU_a becomes idle and invoke newly idle balance
2) Group_b is found as the busiest group
3) CPU_b_1 is found as the busiest CPU, nr_running = 5
4) detach_tasks check CPU_b_1's run queue again, nr_running = 1, goto redo
5) Group_b is still found as the busiest group
6) This time CPU_b_2 is found as the busiest CPU, nr_running = 3
7) detach_tasks successfully, 2 tasks moved.

If we skipped redo,
- CPU_a exit load balance and remain idle
- tasks stay on CPU_b_2's runqueue, wait for the next load balancing

The two tasks could have been moved to the idle CPU and get executed
immediately.

> 
>> 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
> I see.

Thanks,
-Aubrey