linux-kernel - Re: [PATCH 1/2] sched/topology: build_sched

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Date:   Wed, 10 Apr 2019 11:17:15 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org, Dietmar.Eggemann@....com,
        morten.rasmussen@....com
Subject: Re: [PATCH 1/2] sched/topology: build_sched_groups: Skip duplicate
 group rewrites

On 10/04/2019 10:27, Qais Yousef wrote:
[...]
>> @@ -1066,9 +1067,14 @@ static struct sched_group *get_group(int cpu, struct sd_data *sdd)
>>  	sg = *per_cpu_ptr(sdd->sg, cpu);
>>  	sg->sgc = *per_cpu_ptr(sdd->sgc, cpu);
>>  
>> -	/* For claim_allocations: */
>> -	atomic_inc(&sg->ref);
>> -	atomic_inc(&sg->sgc->ref);
>> +	/* Increase refcounts for claim_allocations: */
>> +	already_visited = atomic_inc_return(&sg->ref) > 1;
>> +	/* sgc visits should follow a similar trend as sg */
>> +	WARN_ON(already_visited != (atomic_inc_return(&sg->sgc->ref) > 1));
> 
> NIT: I think it would be better to not have any side effect of calling
> WARN_ON(). Ie: do the atomic_inc_return() outside the WARN_ON() condition.
> Having two bool already_visited_sg and already_visited_sgc will make the code
> more readable too.
> 

I agree it's not the nicest reading flow there is. It's already gone in
tip/sched/core but if needed I can resend with this extra separation.

> Thanks
> 
> --
> Qais Yousef
> 
>> +
>> +	/* If we have already visited that group, it's already initialized. */
>> +	if (already_visited)
>> +		return sg;
>>  
>>  	if (child) {
>>  		cpumask_copy(sched_group_span(sg), sched_domain_span(child));
>> --
>> 2.20.1
>>