Re: [PATCH V6 0/8] blk-mq: improvement CPU hotplug

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Hi Ming,

On Tue, Apr 07, 2020 at 05:28:53PM +0800, Ming Lei wrote:
> Hi,
> 
> Thomas mentioned:
>     "
>      That was the constraint of managed interrupts from the very beginning:
>     
>       The driver/subsystem has to quiesce the interrupt line and the associated
>       queue _before_ it gets shutdown in CPU unplug and not fiddle with it
>       until it's restarted by the core when the CPU is plugged in again.
>     "
> 
> But no drivers or blk-mq do that before one hctx becomes inactive(all
> CPUs for one hctx are offline), and even it is worse, blk-mq stills tries
> to run hw queue after hctx is dead, see blk_mq_hctx_notify_dead().
> 
> This patchset tries to address the issue by two stages:
> 
> 1) add one new cpuhp state of CPUHP_AP_BLK_MQ_ONLINE
> 
> - mark the hctx as internal stopped, and drain all in-flight requests
> if the hctx is going to be dead.
> 
> 2) re-submit IO in the state of CPUHP_BLK_MQ_DEAD after the hctx becomes dead
> 
> - steal bios from the request, and resubmit them via generic_make_request(),
> then these IO will be mapped to other live hctx for dispatch
> 
> Please comment & review, thanks!

FWIW, I've stress test this series by running the stress-cpu-hotplug
with a fio workload in the background. Nothing exploded, all just
worked fine.

Thanks,
Daniel



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