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Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root domain

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On 11/07/16 11:18, Peter Zijlstra wrote:
> On Wed, Jun 22, 2016 at 06:03:17PM +0100, Morten Rasmussen wrote:
>> @@ -6905,11 +6906,19 @@ static int build_sched_domains(const struct cpumask *cpu_map,
>>  	/* Attach the domains */
>>  	rcu_read_lock();
>>  	for_each_cpu(i, cpu_map) {
>> +		rq = cpu_rq(i);
>>  		sd = *per_cpu_ptr(d.sd, i);
>>  		cpu_attach_domain(sd, d.rd, i);
>> +
>> +		if (rq->cpu_capacity_orig > rq->rd->max_cpu_capacity)
>> +			rq->rd->max_cpu_capacity = rq->cpu_capacity_orig;
>>  	}
> 
> Should you not set that _before_ cpu_attach_domain(), such that the
> state is up-to-date when its published?

yes, much better.

> Also, since its lockless, should we not use {READ,WRITE}_ONCE() with it?

You mean for rq->rd->max_cpu_capacity ? IMHO, there is a data dependency
between the read and the write and the code only runs on one cpu.

I assume here that this is related to item 2 'Overlapping loads and
stores within a particular CPU ...' in GUARANTEES of
doc/Documentation/memory-barriers.txt.

Do I miss something?

>>  	rcu_read_unlock();
>>  
>> +	if (rq)
>> +		pr_info("span: %*pbl (max cpu_capacity = %lu)\n",
>> +			cpumask_pr_args(cpu_map), rq->rd->max_cpu_capacity);
>> +
> 
> While a single statement, it is multi line, please add brackets.

OK.

> 
>>  	ret = 0;
>>  error:

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Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root  domain Peter Zijlstra <peterz@infradead.org> - 2016-07-11 12:20 +0200
  Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root  domain Dietmar Eggemann <dietmar.eggemann@arm.com> - 2016-07-11 18:20 +0200
    Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root  domain Peter Zijlstra <peterz@infradead.org> - 2016-07-12 13:50 +0200
      Re: [PATCH v2 06/13] sched: Store maximum per-cpu capacity in root  domain Dietmar Eggemann <dietmar.eggemann@arm.com> - 2016-07-13 13:30 +0200

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