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

From: Jens Axboe
Date: Fri Apr 17 2015 - 11:46:30 EST


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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/