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

[Guenter Roeck <linux@roeck-us.net>]

On Thu, Apr 16, 2015 at 11:32:20PM -0700, Guenter Roeck wrote:
> Hi,
> 
> my qemu-sparc64 tests fail to run with kernel v4.0-7245-ga39ef1a7c609.
> Bisect points to commit 889fa31f00b ("blk-mq: reduce unnecessary software
> queue looping"). Reverting this commit fixes the problem.
> 
> I had a look into the commit, but I have no idea what might be wrong.
> 
> I made the bisect log, images, configuration file, root file system, and directions
> on how to run the images available at http://server.roeck-us.net/qemu/sparc64.
> 
> Please let me know if there is any other information I can provide.
> 
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.

Guenter