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

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Message-ID: <x491u3mpf2b.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 15 Oct 2013 14:25:00 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: blk_mq_update_queue_map makes an (invalid?) assumption about cpu ordering

Jens Axboe <axboe@...nel.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
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