Re: [PATCH] workqueue: don't alloc_percpu for single workqueue

[Oleg Nesterov <oleg@redhat.com>]

On 01/22, Lai Jiangshan wrote:
>
> Oleg Nesterov wrote:
> > On 01/21, Lai Jiangshan wrote:
> >> @@ -906,6 +907,13 @@ void destroy_workqueue(struct workqueue_struct *wq)
> >>  	const struct cpumask *cpu_map = wq_cpu_map(wq);
> >>  	int cpu;
> >>
> >> +	if (is_wq_single_threaded(wq)) {
> >> +		cleanup_workqueue_thread(wq->cpu_wq);
> >> +		kfree(wq->cpu_wq);
> >> +		kfree(wq);
> >> +		return;
> >> +	}
> >
> > again, not sure I understand why this change is needed. Afaics we
> > only need to use kfree(wq->cpu_wq) instead of free_percpu() if
> > it is single-threaded.
> >
>
> I think this change is needed.
> In the single thread case, we don't need
> 1) cpu_maps_update_begin(). --> require cpu_add_remove_lock
> 2) remove workqueue from the list. (we did not inserted it)
>
> It is indeed that there is no bad result occurred when we do these
> things for single thread. But I think the destroying should not
> do things more than the creating.

I disagree.

Firstly, this path is rare and not time critical, it is better
to save a couple of bytes from .text.

But mostly I dislike the fact that we add another special case
for the single-threaded wqs which is not strictly needed.

Following your logic we can also change flush_workqueue(), it
doesn't need for_each_cpu_mask_nr() when single-threaded.


That said, I agree this is a matter of taste, I won't persist.

Oleg.