On Fri, Mar 09, 2018 at 10:24:45AM -0700, Keith Busch wrote:
> On Thu, Mar 08, 2018 at 08:42:20AM +0100, Christoph Hellwig wrote:
> > 
> > So I suspect we'll need to go with a patch like this, just with a way
> > better changelog.
> I have to agree this is required for that use case. I'll run some
> quick tests and propose an alternate changelog.
> Longer term, the current way we're including offline present cpus either
> (a) has the driver allocate resources it can't use or (b) spreads the
> ones it can use thinner than they need to be. Why don't we rerun the
> irq spread under a hot cpu notifier for only online CPUs?

4b855ad371 ("blk-mq: Create hctx for each present CPU") removes handling
mapping change via hot cpu notifier. Not only code is cleaned up, but
also fixes very complicated queue dependency issue:

- loop/dm-rq queue depends on underlying queue
- for NVMe, IO queue depends on admin queue

If freezing queue can be avoided in CPU notifier, it should be fine to
do that, otherwise it need to be avoided.


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