Re: [PATCH RESEND] mm/vmalloc: Use dedicated unbound workqueues for vmap drain

[Hillf Danton <hdanton@sina.com>]

On Wed, 16 Sep 2026 18:00:20 +0200 "Uladzislau Rezki (Sony)" wrote:
> On Wed, Sep 16, 2026 at 08:12:27AM +0800, Hillf Danton wrote:
> > On Mon, 14 Sep 2026 18:56:26 +0200 "Uladzislau Rezki (Sony)" wrote:
> > > This patch does not use queue_work_on() semantic thus i do not want to
> > > queue all helpers on current CPU. Instead scheduler does balancing and
> > > that is it.
> > > 
> > [Fair queue in the Eric Dumazet accent]
> > 
> > +static bool
> > +schedule_drain_vmap_work(struct workqueue_struct *wq,
> > +		struct work_struct *work)
> > +{
> > +	if (wq)
> > +		return queue_work(wq, work);
> > +
> > +	return false;
> > +}
> > +
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/workqueue.h#n697
> > 
> > static inline bool queue_work(struct workqueue_struct *wq,
> > 			      struct work_struct *work)
> > {
> > 	return queue_work_on(WORK_CPU_UNBOUND, wq, work);
> > }
> > 
> > queue_work_on
> >   __queue_work
> > 	if (req_cpu == WORK_CPU_UNBOUND) {
> > 		if (wq->flags & WQ_UNBOUND)
> > 			cpu = wq_select_unbound_cpu(raw_smp_processor_id());
> > 		else
> > 			cpu = raw_smp_processor_id();
> > 	}
> > /*
> >  * When queueing an unbound work item to a wq, prefer local CPU if allowed
> >  * by wq_unbound_cpumask.  Otherwise, round robin among the allowed ones to
> >  * avoid perturbing sensitive tasks.
> >  */
> > static int wq_select_unbound_cpu(int cpu)
> > {
> >  	pr_warn_once("workqueue: round-robin CPU selection forced, expect performance impact\n");
> > }
> > 
> /**
>  * worker_attach_to_pool() - attach a worker to a pool
>  * @worker: worker to be attached
>  * @pool: the target pool
>  *
>  * Attach @worker to @pool.  Once attached, the %WORKER_UNBOUND flag and
>  * cpu-binding of @worker are kept coordinated with the pool across
>  * cpu-[un]hotplugs.
>  */
> static void worker_attach_to_pool(struct worker *worker,
> 				  struct worker_pool *pool)
> {
> 	mutex_lock(&wq_pool_attach_mutex);
> 
> 	/*
> 	 * The wq_pool_attach_mutex ensures %POOL_DISASSOCIATED remains stable
> 	 * across this function. See the comments above the flag definition for
> 	 * details. BH workers are, while per-CPU, always DISASSOCIATED.
> 	 */
> 	if (pool->flags & POOL_DISASSOCIATED) {
> 		worker->flags |= WORKER_UNBOUND;
> 	} else {
> 		WARN_ON_ONCE(pool->flags & POOL_BH);
> 		kthread_set_per_cpu(worker->task, pool->cpu);
> 	}
> ...
> }
> 
> A local CPU preference for WQ_UNBOUND is not the same as executing on
> the __bound__ per-CPU system kworker.
>
As Ulad, like Yu Zhao, is one of the couple black horses I saw in mm the
past a couple years, lad, I make the difference between BOUND and UNBOUND
workers as clear as it is.

Given numa node1 including cpu8-15 without cpu hotplug cared, a bound worker
for cpu9 can not migrate to any other cpu, while a unbound worker can run on
any cpu of node1, that is all.

Important UN/BOUND have nothing to do with eevdf (and balancing cpus) because
of different layers.

And at best I suspect what you missed is the difference between drain_vmap_work
and lru_add_drain_work, but I do not like the latter as it annoyed the RT/full
nohz apps more than thought [11].

static DECLARE_WORK(drain_vmap_work, drain_vmap_area_work);
static DEFINE_PER_CPU(struct work_struct, lru_add_drain_work);

[11] Subject: [PATCH v4 3/4] swap: apply new pw_queue_on() interface
https://lore.kernel.org/lkml/20260519012754.240804-4-leobras.c@gmail.com/