Re: [PATCH v3 06/10] sched/fair: Use the prefer_sibling flag of the current sched domain

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 16/02/2023 06:21, Ricardo Neri wrote:
> On Mon, Feb 13, 2023 at 10:43:28PM -0800, Ricardo Neri wrote:
>> On Mon, Feb 13, 2023 at 01:17:09PM +0100, Dietmar Eggemann wrote:
>>> On 10/02/2023 19:31, Ricardo Neri wrote:
>>>> On Fri, Feb 10, 2023 at 05:12:30PM +0000, Valentin Schneider wrote:
>>>>> On 10/02/23 17:53, Peter Zijlstra wrote:
>>>>>> On Fri, Feb 10, 2023 at 02:54:56PM +0000, Valentin Schneider wrote:

[...]

>>> Can you not detect the E-core dst_cpu case on MC with:
>>>
>>> +       if (child)
>>> +               sds->prefer_sibling = child->flags & SD_PREFER_SIBLING;
>>> +       else if (sds->busiest)
>>> +               sds->prefer_sibling = sds->busiest->group_weight > 1;
>>
>> Whose child wants the prefer_sibling setting? In update_sd_lb_stats(), it
>> is set based on the flags of the destination CPU's sched domain. But when
>> used in find_busiest_group() tasks are spread from the busiest group's
>> child domain.
>>
>> Your proposed code, also needs a check for SD_PREFER_SIBLING, no?
> 
> I tweaked the solution that Dietmar proposed:
> 
> -	sds->prefer_sibling = child && child->flags & SD_PREFER_SIBLING;
> +	if (sds->busiest)
> +		sds->prefer_sibling = sds->busiest->flags & SD_PREFER_SIBLING;

Maybe:

sds->prefer_sibling = !!(sds->busiest->flags & SD_PREFER_SIBLING);

1 vs 2048 ?

> This comes from the observation that the prefer_sibling setting acts on
> busiest group. It then depends on whether the busiest group, not the local
> group, has child sched sched domains. Today it works because in most cases
> both the local and the busiest groups have child domains with the SD_
> PREFER_SIBLING flag.
> 
> This would also satisfy sched domains with the SD_ASYM_CPUCAPACITY flag as
> prefer_sibling would not be set in that case.
> 
> It would also conserve the current behavior at the NUMA level. We would
> not need to implement SD_SPREAD_TASKS.
> 
> This would both fix the SMT vs non-SMT bug and be less invasive.

Yeah, much better! I always forget that we have those flags on SGs now
as well. Luckily, we just need to check busiest sg to cover all cases.