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

Hannes Reinecke
2025-09-08T07:51:39Z

On 9/8/25 09:26, Daniel Wagner wrote:
> 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.

Oh, that's okay. I'm perfectly fine to update the cpumask on every call.
What makes me wonder is the initialisation of blk_hk_online_mask;
it's zeroed _at boot_, and then all we do is calling cpumask_and()
for every invocation. So the mask will only increase in scope,
and never decrease.

Wouldn't it be better to call 'cpumask_zero' before 'cpumask_and'?

Cheers,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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