Re: [RFC v5 2/3] workqueue: Add support for real-time workers

[Tejun Heo <tj@kernel.org>]

Hello,

On Wed, Sep 23, 2026 at 05:12:50PM +0100, Tvrtko Ursulin wrote:
> For use cases such as the DRM scheduler submitting work to the GPU on
> behalf of low latency userspace applications, where latter have sufficient
> privileges to have had successfully obtained realtime Vulkan global
> priority, competing with random background CPU load can create large
> latency spikes which gets in the way of a smooth user experience.

panthor's group_priority_permit() also allows realtime groups for DRM
master without CAP_SYS_NICE. What's the usage model there? Should DRM
master be enough to get RT workers?

> @@ -374,8 +374,9 @@ enum wq_flags {
>  	WQ_FREEZABLE		= 1 << 2, /* freeze during suspend */
>  	WQ_MEM_RECLAIM		= 1 << 3, /* may be used for memory reclaim */
>  	WQ_HIGHPRI		= 1 << 4, /* high priority */
> -	WQ_CPU_INTENSIVE	= 1 << 5, /* cpu intensive workqueue */
> -	WQ_SYSFS		= 1 << 6, /* visible in sysfs, see workqueue_sysfs_register() */
> +	WQ_RTPRI		= 1 << 5, /* real-time priority, valid only with WQ_UNBOUND */
> +	WQ_CPU_INTENSIVE	= 1 << 6, /* cpu intensive workqueue */
> +	WQ_SYSFS		= 1 << 7, /* visible in sysfs, see workqueue_sysfs_register() */

Can we name it just WQ_RT? Also, I think WQ_RT is closer to WQ_BH. We can
reorder the flags later if that helps but for now can you just put it in an
empty slot?

> @@ -127,6 +127,7 @@ enum wq_internal_consts {
>  	 */
>  	RESCUER_NICE_LEVEL	= MIN_NICE,
>  	HIGHPRI_NICE_LEVEL	= MIN_NICE,
> +	RTPRI_NICE_LEVEL	= MIN_NICE - 1,

Can we match the scheduler's representation instead by replacing
attrs->nice with attrs->prio which uses the same encoding as p->prio?
Normal pools would be NICE_TO_PRIO(nice) and WQ_RT pools would be in the RT
range. create_worker() would then do:

	if (rt_prio(pool->attrs->prio))
		sched_set_fifo_low(worker->task);
	else
		set_user_nice(worker->task, PRIO_TO_NICE(pool->attrs->prio));

> @@ -7606,7 +7626,10 @@ static ssize_t nice_show(struct device *dev, struct device_attribute *attr,
>  	int written;
>
>  	mutex_lock(&wq->mutex);
> -	written = scnprintf(buf, PAGE_SIZE, "%d\n", wq->attrs->nice);
> +	if (wq->attrs->nice == RTPRI_NICE_LEVEL)
> +		written = scnprintf(buf, PAGE_SIZE, "rt\n");
> +	else
> +		written = scnprintf(buf, PAGE_SIZE, "%d\n", wq->attrs->nice);

Can you also show rt in pr_cont_pool_info() and tools/workqueue/wq_dump.py?

Thanks.

-- 
tejun