Re: [PATCH v4 8/9] blk-mq: use hk cpus only when isolcpus=managed_irq is enabled

[Daniel Wagner <dwagner@suse.de>]

On Thu, Dec 19, 2024 at 05:20:44PM +0800, Ming Lei wrote:
> > +	cpumask_andnot(isol_mask,
> > +		       cpu_possible_mask,
> > +		       housekeeping_cpumask(HK_TYPE_MANAGED_IRQ));
> > +
> > +	for_each_cpu(cpu, isol_mask) {
> > +		qmap->mq_map[cpu] = qmap->queue_offset + queue;
> > +		queue = (queue + 1) % qmap->nr_queues;
> > +	}
> 
> Looks the IO hang issue in V3 isn't addressed yet, is it?
> 
> https://lore.kernel.org/linux-block/ZrtX4pzqwVUEgIPS@fedora/

I've added an explanation in the cover letter why this is not
addressed. From the cover letter:

I've experimented for a while and all solutions I came up were horrible
hacks (the hotpath needs to be touched) and I don't want to slow down all
other users (which are almost everyone). IMO, it's just not worth trying
to fix this corner case. If the user is using isolcpus and does CPU
hotplug, we can expect that the user can also first offline the isolated
CPUs. I've discussed this topic during ALPSS and the room came to the
same conclusion. Thus I just added a patch which issues a warning that
IOs are likely to hang.