Re: boot stall regression due to blk-mq: use percpu_ref for mq usage count

Tejun Heo
2014-09-23T06:03:46Z

On Tue, Sep 23, 2014 at 02:01:41AM -0400, Tejun Heo wrote:
> On Tue, Sep 23, 2014 at 07:59:24AM +0200, Christoph Hellwig wrote:
> > "[PATCHSET percpu/for-3.18] percpu_ref: implement switch_to_atomic/percpu()"
> >
> > looks way to big for 3.17, and the regression was introduced in the 3.17
> > merge window. I'm not sure what was broken before, but it defintively
> > survived a lot of testing.
>
> Do we even care about fixing it for 3.17? scsi-mq isn't enabled by
> default even for 3.18. The open-coded percpu ref thing was subtly
> broken there. It'd be difficult to trigger but I'm fairly sure it'd
> crap out in the wild once in a blue moon.

Hmmm... also, this is actually an issue with scsi that block layer is
working around. For other drivers (virtio), the latency during
shutdown should be completely fine.

--
tejun