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:
