Re: [RFC PATCH 3/3] workqueue: Enable unbound cpumask update on ordered workqueues

[Waiman Long <longman@redhat.com>]

On 1/31/24 12:00, Tejun Heo wrote:
> Hello,
>
> On Tue, Jan 30, 2024 at 01:33:36PM -0500, Waiman Long wrote:
>> +/* requeue the work items stored in wq->o_list */
>> +static void requeue_ordered_works(struct workqueue_struct *wq)
>> +{
>> +	LIST_HEAD(head);
>> +	struct work_struct *work, *next;
>> +
>> +	raw_spin_lock_irq(&wq->o_lock);
>> +	if (list_empty(&wq->o_list))
>> +		goto unlock_out;	/* No requeuing is needed */
>> +
>> +	list_splice_init(&wq->o_list, &head);
>> +	raw_spin_unlock_irq(&wq->o_lock);
>> +
>> +	/*
>> +	 * Requeue the first batch of work items. Since it may take a while
>> +	 * to drain the old pwq and update the workqueue attributes, there
>> +	 * may be a rather long list of work items to process. So we allow
>> +	 * queue_work() callers to continue putting their work items in o_list.
>> +	 */
>> +	list_for_each_entry_safe(work, next, &head, entry) {
>> +		list_del_init(&work->entry);
>> +		local_irq_disable();
>> +		__queue_work_rcu_locked(WORK_CPU_UNBOUND, wq, work);
>> +		local_irq_enable();
>> +	}
>> +
>> +	/*
>> +	 * Now check if there are more work items queued, if so set ORD_WAIT
>> +	 * and force incoming queue_work() callers to busy wait until the 2nd
>> +	 * batch of work items have been properly requeued. It is assumed
>> +	 * that the 2nd batch should be much smaller.
>> +	 */
>> +	raw_spin_lock_irq(&wq->o_lock);
>> +	if (list_empty(&wq->o_list))
>> +		goto unlock_out;
>> +	WRITE_ONCE(wq->o_state, ORD_WAIT);
>> +	list_splice_init(&wq->o_list, &head);
>> +	raw_spin_unlock(&wq->o_lock);	/* Leave interrupt disabled */
>> +	list_for_each_entry_safe(work, next, &head, entry) {
>> +		list_del_init(&work->entry);
>> +		__queue_work_rcu_locked(WORK_CPU_UNBOUND, wq, work);
>> +	}
>> +	WRITE_ONCE(wq->o_state, ORD_NORMAL);
>> +	local_irq_enable();
>> +	return;
>> +
>> +unlock_out:
>> +	WRITE_ONCE(wq->o_state, ORD_NORMAL);
>> +	raw_spin_unlock_irq(&wq->o_lock);
>> +}
> I'm not a big fan of this approach. It's a rather big departure from how
> things are usually done in workqueue. I'd much prefer sth like the
> following:
>
> - Add the ability to mark an unbound pwq plugged. If plugged,
>     pwq_tryinc_nr_active() always fails.
>
> - When cpumasks need updating, set max_active of all ordered workqueues to
>    zero and flush them. Note that if you set all max_actives to zero (note
>    that this can be another "plug" flag on the workqueue) first, all the
>    ordered workqueues would already be draining, so calling flush_workqueue()
>    on them sequentially shouldn't take too long.
>
> - Do the normal pwq allocation and linking but make sure that all new
>    ordered pwqs start plugged.
>
> - When update is done, restore the max_actives on all ordered workqueues.
>
> - New work items will now get queued to the newest dfl_pwq which is plugged
>    and we know that wq->pwqs list contain pwqs in reverse creation order. So,
>    from pwq_release_workfn(), if the pwq being released is for an ordered
>    workqueue and not plugged, unplug the pwq right in front.
>
> This hopefully should be less invasive.
>
> Thanks.

Thanks for suggestion. I will rework the patch series to use this approach.

Cheers,
Longman