Hi Jens,

> Thanks for sending this, very interesting! As per this email, I took a
> look at the NUMA bindings. If you can, please try this one-liner below.
> I'd be interested to know if that removes the fluctuations you're seeing
> due to bad locality.
> 
> Looks like kthread_create_on_node() doesn't actually do anything (at
> least in terms of binding).
>
> 
> diff --git a/fs/io-wq.c b/fs/io-wq.c
> index 74b84e8562fb..7bebb198b3df 100644
> --- a/fs/io-wq.c
> +++ b/fs/io-wq.c
> @@ -676,6 +676,7 @@ static bool create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index)
>  		kfree(worker);
>  		return false;
>  	}
> +	kthread_bind_mask(worker->task, cpumask_of_node(wqe->node));
>  
>  	raw_spin_lock_irq(&wqe->lock);
>  	hlist_nulls_add_head_rcu(&worker->nulls_node, &wqe->free_list);
> 

I no longer have access to that system, but I guess it will help, thanks!

With this:

        worker->task = kthread_create_on_node(io_wqe_worker, worker, wqe->node,
                                "io_wqe_worker-%d/%d", index, wqe->node);

I see only "io_wqe_worker-0" and "io_wqe_worker-1" in top, without '/0' or '/1' at the end,
this is because set_task_comm() truncates to 15 characters.

As developer I think 'io_wqe' is really confusing, just from reading I thought it
means "work queue entry", but it's a per numa node worker pool container...
'struct io_wq_node *wqn' would be easier to understand for me...

Would it make sense to give each io_wq a unique identifier and use names like this:
(fdinfo of the io_uring fd could also include the io_wq id)

 "io_wq-%u-%u%c", wq->id, wqn->node, index == IO_WQ_ACCT_BOUND ? 'B' : 'U')

 io_wq-500-M
 io_wq-500-0B
 io_wq-500-0B
 io_wq-500-1B
 io_wq-500-0U
 io_wq-200-M
 io_wq-200-0B
 io_wq-200-0B
 io_wq-200-1B
 io_wq-200-0U

I'm not sure how this interacts with workers moving between bound and unbound
and maybe a worker id might also be useful (or we rely on their pid)

I just found that proc_task_name() handles PF_WQ_WORKER special
and cat /proc/$pid/comm can expose something like:
  kworker/u17:2-btrfs-worker-high

ps and top still truncate, but that can be fixed.

Some workqueues also expose their details under /sys/bus/workqueue/.

I guess there's a lot of potential to optimize the details exposed to the admins
and (userspace) developers.

metze