linux-kernel - Re: [PATCH] sched/topology: Fix overlapping sched

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Date:   Wed, 25 Mar 2020 18:09:44 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, mingo@...nel.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, morten.rasmussen@....com,
        mgorman@...hsingularity.net
Subject: Re: [PATCH] sched/topology: Fix overlapping sched_group build


On Wed, Mar 25 2020, Valentin Schneider wrote:
> On Tue, Mar 24 2020, Valentin Schneider wrote:
>>  kernel/sched/topology.c | 23 ++++++++++++++++++++---
>>  1 file changed, 20 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>> index 8344757bba6e..7033b27e5162 100644
>> --- a/kernel/sched/topology.c
>> +++ b/kernel/sched/topology.c
>> @@ -866,7 +866,7 @@ build_balance_mask(struct sched_domain *sd, struct sched_group *sg, struct cpuma
>>                       continue;
>>
>>               /* If we would not end up here, we can't continue from here */
>> -		if (!cpumask_equal(sg_span, sched_domain_span(sibling->child)))
>> +		if (!cpumask_subset(sg_span, sched_domain_span(sibling->child)))
>
> So this is one source of issues; what I've done here is a bit stupid
> since we include CPUs that *cannot* end up there. What I should've done
> is something like:
>
>   cpumask_and(tmp, sched_domain_span(sibling->child), sched_domain_span(sd));
>   if (!cpumask_equal(sg_span, tmp))
>       ...
>
> But even with that I just unfold even more horrors: this breaks the
> overlapping sched_group_capacity (see 1676330ecfa8 ("sched/topology: Fix
> overlapping sched_group_capacity")).
>
> For instance, here I would have
>
>   CPU0-domain2-group4: span=4-5
>   CPU4-domain2-group4: span=4-7 mask=4-5
>

^ That's using Dietmar's qemu setup; on the D06 that is

  CPU0-domain2-group48: span=48-71
  CPU48-domain2-group48: span=48-95 mask=48-71

> Both groups are at the same topology level and have the same first CPU,
> so they point to the same sched_group_capacity structure - but they
> don't have the same span. They would without my "fix", but then the
> group spans are back to being wrong. I'm starting to think this is
> doomed, at least in the current state of things :/