Re: Samba with multichannel and io_uring

[Jens Axboe <axboe@kernel.dk>]

On 10/16/20 5:49 AM, Stefan Metzmacher wrote:
> 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!

I queued up it when I sent it out, and it'll go into stable as well.
I since verified on NUMA here that it does the right thing, and that
things weren't affinitized properly before. So pretty confident that it
will indeed solve this issue!

> 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 don't think that's too important, as it's just a snapshot in time. So
it'll fluctuate based on the role of the worker.

> 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

Yep, that's how they do fancier names. It's been on my agenda for a while
to do something about this, I'll try and cook something up for 5.11.

-- 
Jens Axboe