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During CPU offline, in blk_mq_hctx_notify_offline(),
blk_mq_hctx_has_online_cpu() returns true even though the last cpu in
hctx 0 is offline because isolated cpus join hctx 0 unexpectedly, so IOs in
hctx 0 won't be drained.

However managed irq core code still shutdowns the hw queue's irq because all
CPUs in this hctx are offline now. Then IO hang is triggered, isn't it?

The current blk-mq takes static & global queue/CPUs mapping, in which all CPUs
are covered. This patchset removes isolated CPUs from the mapping, and the
change is big from viewpoint of blk-mq queue mapping.

> 
> > that means one random hctx(or even NULL) may be used for submitting
> > IO from isolated CPUs,
> > then there can be io hang risk during cpu hotplug, or
> > kernel panic when submitting bio.
> 
> Can you elaborate a bit more? I must miss something important here.
> 
> Anyway, my understanding is that when the last CPU of a hctx goes
> offline the affinity is broken and assigned to an online HK CPU. And we
> ensure all flight IO have finished and also ensure we don't submit any
> new IO to a CPU which goes offline.
> 
> FWIW, I tried really hard to get an IO hang with cpu hotplug.
 
Please see above.


thanks,
Ming





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