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

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Date:	Tue, 15 Oct 2013 12:14:03 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: blk_mq_update_queue_map makes an (invalid?) assumption about cpu
 ordering

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

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