[PATCH v9 10/13] blk-mq: use hk cpus only when isolcpus=io_queue is enabled

[Aaron Tomlin]

On Tue, Mar 31, 2026 at 05:05:07PM -0600, Keith Busch wrote:
> > +static bool blk_mq_validate(struct blk_mq_queue_map *qmap,
> > +			    const struct cpumask *active_hctx)
> > +{
> > +	/*
> > +	 * Verify if the mapping is usable when housekeeping
> > +	 * configuration is enabled
> > +	 */
> > +
> > +	for (int queue = 0; queue < qmap->nr_queues; queue++) {
> > +		int cpu;

Hi Keith,

Thank you for taking the time to review and test.

> > +
> > +		if (cpumask_test_cpu(queue, active_hctx)) {
> > +			/*
> > +			 * This htcx has at least one online CPU thus it
> 
> Typo, should say "hctx".

Acknowledged.

> > +			 * is able to serve any assigned isolated CPU.
> > +			 */
> > +			continue;
> > +		}
> > +
> > +		/*
> > +		 * There is no housekeeping online CPU for this hctx, all
> > +		 * good as long as all non houskeeping CPUs are also
> 
> Typo, "housekeeping".

Acknowledged.

> ...
> 
> >  void blk_mq_map_queues(struct blk_mq_queue_map *qmap)
> >  {
> > -	const struct cpumask *masks;
> > +	struct cpumask *masks __free(kfree) = NULL;
> > +	const struct cpumask *constraint;
> >  	unsigned int queue, cpu, nr_masks;
> > +	cpumask_var_t active_hctx;
> >  
> > -	masks = group_cpus_evenly(qmap->nr_queues, &nr_masks);
> > -	if (!masks) {
> > -		for_each_possible_cpu(cpu)
> > -			qmap->mq_map[cpu] = qmap->queue_offset;
> > -		return;
> > -	}
> > +	if (!zalloc_cpumask_var(&active_hctx, GFP_KERNEL))
> > +		goto fallback;
> > +
> > +	if (housekeeping_enabled(HK_TYPE_IO_QUEUE))
> > +		constraint = housekeeping_cpumask(HK_TYPE_IO_QUEUE);
> > +	else
> > +		constraint = cpu_possible_mask;
> > +
> > +	/* Map CPUs to the hardware contexts (hctx) */
> > +	masks = group_mask_cpus_evenly(qmap->nr_queues, constraint, &nr_masks);
> > +	if (!masks)
> > +		goto free_fallback;
> >  
> >  	for (queue = 0; queue < qmap->nr_queues; queue++) {
> > -		for_each_cpu(cpu, &masks[queue % nr_masks])
> > -			qmap->mq_map[cpu] = qmap->queue_offset + queue;
> > +		unsigned int idx = (qmap->queue_offset + queue) % nr_masks;
> > +
> > +		for_each_cpu(cpu, &masks[idx]) {
> > +			qmap->mq_map[cpu] = idx;
> 
> I think there's something off with this when we have multiple queue maps. The
> wrapping loses the offset when we've isolated CPUs, so I think the index would
> end up incorrect.
> 
> Trying this series out when "nvme.poll_queues=2" with isolcpus set, I am
> getting a kernel panic:

To resolve this comprehensively, I have made the following adjustments:

    1.  qmap->mq_map now strictly stores the absolute hardware index
        (qmap->queue_offset + queue) across both the primary assignment
        loop and the unassigned CPU fallback loop.

    2.  The active_hctx cpumask now strictly tracks the relative index
        (queue) for the current map being processed, preventing
        out-of-bounds mask tracking.

    3.  The validation check within blk_mq_validate() has been updated to
        correctly compare the absolute index stored in mq_map against the
        offset-adjusted queue index.

I shall fold these corrections directly into the next series.


Best regards,
-- 
Aaron Tomlin
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