On 08/29/2016 12:06 PM, Gabriel Krisman Bertazi wrote:

Jens Axboe [off-list ref] writes:

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Can you try this patch? It's not perfect, but I'll be interested if it
makes a difference for you.

Hi Jens,

Sorry for the delay.  I just got back to this and have been running your
patch on top of 4.8 without a crash for over 1 hour.  I wanna give it
more time to make sure it's running properly, though.

Let me get back to you after a few more rounds of test.

Thanks, sounds good. The patches have landed in mainline too.

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This one should handle the WARN_ON() for running the hw queue on the
wrong CPU as well.

On the workaround you added to prevent WARN_ON, we surely need to
prevent blk_mq_hctx_next_cpu from scheduling dead cpus in the first
place, right..  How do you feel about the following RFC?  I know it's
not a complete fix, but it feels like a good improvement to me.

http://www.spinics.net/lists/linux-scsi/msg98608.html

But we can't completely prevent it, and I don't think we have to. I just
don't want to trigger a warning for something that's a valid condition.
I want the warning to trigger if this happens without the CPU going
offline, since then it's indicative of a real bug in the mapping. Your
patch isn't going to prevent it either - it'll shrink the window, at the
expense of making blk_mq_hctx_next_cpu() more expensive. So I don't
think it's worthwhile.


-- 
Jens Axboe