linux-kernel - Re: Upstream kernel fails to run on qemu-sparc64 due to commit 889fa31f0 (blk-mq: reduce unnecessary software queue looping)

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Date:	Fri, 17 Apr 2015 09:46:05 -0600
From:	Jens Axboe <axboe@...com>
To:	Guenter Roeck <linux@...ck-us.net>
CC:	<linux-kernel@...r.kernel.org>,
	Chong Yuan <chong.yuan@...blaze.com>,
	Wenbo Wang <wenbo.wang@...blaze.com>
Subject: Re: Upstream kernel fails to run on qemu-sparc64 due to commit 889fa31f0
 (blk-mq: reduce unnecessary software queue looping)

On 04/17/2015 09:31 AM, Guenter Roeck wrote:
> Hi Jens,
>
> On Fri, Apr 17, 2015 at 08:26:11AM -0600, Jens Axboe wrote:
>>>>
>>> As additional information:
>>>
>>> +                * Set the map size to the number of mapped software queues.
>>> +                * This is more accurate and more efficient than looping
>>> +                * over all possibly mapped software queues.
>>> +                */
>>> +               map->map_size = hctx->nr_ctx / map->bits_per_word;
>>>
>>> On my system, hctx->nr_ctx is 1, and map->bits_per_word is 8.
>>> Thus map->map_size is set to 0, which doesn't make much sense.
>>
>>
>>> The system comes up if I replace the above code with
>>> 		map->map_size = DIV_ROUND_UP(hctx->nr_ctx, map->bits_per_word);
>>>
>>> I have no idea if that is the correct fix, though.
>>
>> Ugh, yes indeed, looks like the <= was lost from a previous patch. Now I
>> wonder why it I didn't see any hangs with this... Thanks for reporting, I'll
>> get a fix in today.
>>
> Assuming that nr_ctx reflects the number of (online) CPUs, my guess is that
> you may have a multiple of bits_per_word CPUs in your system.

Ah yes, now it makes sense. Smallest box I have is 8 CPUs, and generally 
map bits_per_word is in the 5-6 range. So it ends up working out for my 
case, ->map_size would be >= 1.

-- 
Jens Axboe

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