blk_mq_update_queue_map makes an (invalid?) assumption about cpu ordering

blk_mq_update_queue_map makes an (invalid?) assumption about cpu ordering

[Jeff Moyer]

Hi, Jens,

blk_mq_update_queue_map does this:

        for_each_possible_cpu(i) {
                if (!cpu_online(i)) {
                        map[i] = 0;
                        continue;
                }

...
                first_sibling = get_first_sibling(i);
                if (first_sibling == i) {
                        map[i] = cpu_to_queue_index(nr_uniq_cpus, nr_queues,
                                                        queue);
                        queue++;
                } else
                        map[i] = map[first_sibling];

This assumes that the first_sibling is listed before any other siblings,
which I don't believe is true.  I don't think you get any guaranteed
ordering in that cpu_possible_mask.

... or did I miss something?

Cheers,
Jeff

Re: blk_mq_update_queue_map makes an (invalid?) assumption about cpu ordering

[Jens Axboe]

On Tue, Oct 15 2013, Jeff Moyer wrote:
> Hi, Jens,
> 
> blk_mq_update_queue_map does this:
> 
>         for_each_possible_cpu(i) {
>                 if (!cpu_online(i)) {
>                         map[i] = 0;
>                         continue;
>                 }
> 
> ...
>                 first_sibling = get_first_sibling(i);
>                 if (first_sibling == i) {
>                         map[i] = cpu_to_queue_index(nr_uniq_cpus, nr_queues,
>                                                         queue);
>                         queue++;
>                 } else
>                         map[i] = map[first_sibling];
> 
> This assumes that the first_sibling is listed before any other siblings,
> which I don't believe is true.  I don't think you get any guaranteed
> ordering in that cpu_possible_mask.
> 
> ... or did I miss something?

That's correct, it's assuming the first sibling is the lowest numbered
one. Are there cases where that would not be correct? I was sort of
assuming that was what "first" meant here.

-- 
Jens Axboe

Re: blk_mq_update_queue_map makes an (invalid?) assumption about cpu ordering

[Jeff Moyer]

Jens Axboe <axboe@kernel.dk> writes:

>> This assumes that the first_sibling is listed before any other siblings,
>> which I don't believe is true.  I don't think you get any guaranteed
>> ordering in that cpu_possible_mask.
>> 
>> ... or did I miss something?
>
> That's correct, it's assuming the first sibling is the lowest numbered
> one. Are there cases where that would not be correct? I was sort of
> assuming that was what "first" meant here.

Yeah, you're right.  I hadn't read down the call chain:

static int get_first_sibling(unsigned int cpu)
        ret = cpumask_first(topology_thread_cpumask(cpu));

Nothing to see here, move along...

-Jeff