Re: [PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload

[Pierre Gondois <pierre.gondois@arm.com>]

On 11/29/23 09:55, Dietmar Eggemann wrote:
> On 24/11/2023 16:33, Pierre Gondois wrote:
>> Running n CPU-bound tasks on an n CPUs platform:
>> - with asymmetric CPU capacity
>> - not having SD_SHARE_PKG_RESOURCES flag set at the DIE
> 
> nit: DIE is now called PKG on tip sched/core.
> 
> f577cd57bfaa - sched/topology: Rename 'DIE' domain to 'PKG' (2023-10-12
> Peter Zijlstra)
> 
>>    sched domain level (i.e. not DynamIQ systems)
>> might result in a task placement where two tasks run on a big CPU
>> and none on a little CPU. This placement could be more optimal by
>> using all CPUs.
>>
>> Testing platform:
>> Juno-r2:
>> - 2 big CPUs (1-2), maximum capacity of 1024
>> - 4 little CPUs (0,3-5), maximum capacity of 383
>>
>> Testing workload ([1]):
>> Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks
>> is affine to a CPU, except for:
>> - one little CPU which is left idle.
>> - one big CPU which has 2 tasks affine.
>> After the 100ms (step 2), remove the cpumask affinity.
>>
>> Before patch:
>> During step 2, the load balancer running from the idle CPU tags sched
>> domains as:
>> - little CPUs: 'group_has_spare'. Indeed, 3 CPU-bound tasks run on a
>>    4 CPUs sched-domain, and the idle CPU provides enough spare
>>    capacity.
> 
> What is meant by 'idle CPU provides enough spare capacity? I thought the
> task (util_avg ~ 512_ does not fit on the sched group [1,3-5] when we
> consider util_avg/capacity (383)

Right, I meant that when evaluating the 'group_type', there is enough spare
capacity when summing the utilization of CPUs in the the MC sched domain:

---
group_has_capacity()
{
	[...]
	if ((sgs->group_capacity * 100) >
			(sgs->group_util * imbalance_pct))
		return true;
	[...]
}
---

> 
> The calculated imbalance of ~350 is too small for the task-size and
> that's why we need the 'shr_bound(util, env->sd->nr_balance_failed)' to
> let the task load-balance if nr_balance_failed = 2?

Yes exact, the tasks are too big and cannot fit this imbalance value
(representing the available spare capacity in the little CPUs in this case).

'shr_bound(...)' allows to progressively reduce the size of the tasks and
allow migrations after having tried to balance 'nr_balance_failed' times.


> 
> [...]
> 
>> Similar issue reported at:
>> https://lore.kernel.org/lkml/20230716014125.139577-1-qyousef@layalina.io/
>>
>> v1:
>> https://lore.kernel.org/all/20231110125902.2152380-1-pierre.gondois@arm.com/
>>
>> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
>> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
> 
> Even though this cures only classical big.LITTLE it might have a
> positive effect on today's Arm DynamIQ Android systems with Phantom SDs
> when running benchmarks like Geekbench.
> 
> [...]
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
>