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>]

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.

Guenter