Re: [PATCH v8 10/12] blk-mq: use hk cpus only when isolcpus=io_queue is enabled

Daniel Wagner
2025-09-08T07:26:08Z

On Mon, Sep 08, 2025 at 08:13:31AM +0200, Hannes Reinecke wrote:
> > const struct cpumask *blk_mq_online_queue_affinity(void)
> > {
> > + if (housekeeping_enabled(HK_TYPE_IO_QUEUE)) {
> > + cpumask_and(&blk_hk_online_mask, cpu_online_mask,
> > + housekeeping_cpumask(HK_TYPE_IO_QUEUE));
> > + return &blk_hk_online_mask;
>
> Can you explain the use of 'blk_hk_online_mask'?
> Why is a static variable?

The blk_mq_*_queue_affinity helpers return a const struct cpumask *, the
caller doesn't need to free the return value. Because cpumask_and needs
store its result somewhere, I opted for the global static variable.

> To my untrained eye it's being recalculated every time one calls
> this function. And only the first invocation run on an empty mask,
> all subsequent ones see a populated mask.

The cpu_online_mask might change over time, it's not a static bitmap.
Thus it's necessary to update the blk_hk_online_mask. Doing some sort of
caching is certainly possible. Given that we have plenty of cpumask
logic operation in the cpu_group_evenly code path later, I am not so
sure this really makes a huge difference.