Re: [PATCH 05/15] block: simplify elevator reset for updating nr_hw_queues

[Ming Lei <ming.lei@redhat.com>]

On Thu, Apr 10, 2025 at 05:34:18PM +0200, Christoph Hellwig wrote:
> On Thu, Apr 10, 2025 at 09:30:17PM +0800, Ming Lei wrote:
> > @@ -5071,9 +4984,15 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
> >  		blk_mq_debugfs_register_hctxs(q);
> >  	}
> >  
> > -switch_back:
> > -	list_for_each_entry(q, &set->tag_list, tag_set_list)
> > -		blk_mq_elv_switch_back(&head, q);
> > +	list_for_each_entry(q, &set->tag_list, tag_set_list) {
> > +		const char *name = "none";
> > +
> > +		mutex_lock(&q->elevator_lock);
> > +		if (q->elevator && !blk_queue_dying(q))
> > +			name = q->elevator->type->elevator_name;
> > +		__elevator_change(q, name, true);
> > +		mutex_unlock(&q->elevator_lock);
> > +	}
> 
> Coming back to this after looking through the next patches.
> 
> Why do we even need the __elevator_change call here?  We've not
> actually disabled the elevator, and we prevent other callers
> from changing it.
> 
> As you pass in the force argument this now always calls
> elevator_switch and thus blk_mq_init_sched.  But why?

sched tags is built over hctx and depends on ->nr_hw_queues,
when nr_hw_queues is changed, sched tags has to be rebuilt.


Thanks, 
Ming