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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Message-ID: <20150417153136.GA11202@roeck-us.net>
Date:	Fri, 17 Apr 2015 08:31:36 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Jens Axboe <axboe@...com>
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)

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
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