Re: [PATCH] loop: inherit the ioprio in loop woker thread

[Jens Axboe]

On 5/23/24 8:52 AM, yunlong xing wrote:
> Jens Axboe <axboe@xxxxxxxxx> ?2024?5?23??? 21:04???
>>
>> On 5/23/24 12:04 AM, yunlong xing wrote:
>>> Bart Van Assche <bvanassche@xxxxxxx> ?2024?5?23??? 02:12???
>>>>
>>>> On 5/22/24 10:57, Jens Axboe wrote:
>>>>> On 5/22/24 11:38 AM, Bart Van Assche wrote:
>>>>>> On 5/22/24 00:48, Yunlong Xing wrote:
>>>>>>> @@ -1913,6 +1921,10 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
>>>>>>>            set_active_memcg(old_memcg);
>>>>>>>            css_put(cmd_memcg_css);
>>>>>>>        }
>>>>>>> +
>>>>>>> +    if (ori_ioprio != cmd_ioprio)
>>>>>>> +        set_task_ioprio(current, ori_ioprio);
>>>>>>> +
>>>>>>>     failed:
>>>>>>>        /* complete non-aio request */
>>>>>>>        if (!use_aio || ret) {
>>>>>>
>>>>>> Does adding this call in the hot path have a measurable performance impact?
>>>>>
>>>>> It's loop, I would not be concerned with overhead. But it does look pretty
>>>>> bogus to modify the task ioprio from here.
>>>>
>>>> Hi Jens,
>>>>
>>>> Maybe Yunlong uses that call to pass the I/O priority to the I/O submitter?
>>>>
>>>> I think that it is easy to pass the I/O priority to the kiocb submitted by
>>>> lo_rw_aio() without calling set_task_ioprio().
>>>>
>>>> lo_read_simple() and lo_write_simple() however call vfs_iter_read() /
>>>> vfs_iter_write(). This results in a call of do_iter_readv_writev() and
>>>> init_sync_kiocb(). The latter function calls get_current_ioprio(). This is
>>>> probably why the set_task_ioprio() call has been added?
>>>
>>> Yeah that's why I call set_task_ioprio.  I want to the loop kwoker
>>> task?submit I/O to the real disk device?can pass the iopriority of the
>>> loop device request? both lo_rw_aio() and
>>> lo_read_simple()/lo_write_simple().
>>
>> And that's a totally backwards and suboptimal way to do it. The task
>> priority is only used as a last resort lower down, if the IO itself
>> hasn't been appropriately marked.
>>
>> Like I said, it's back to the drawing board on this patch, there's no
>> way it's acceptable in its current form.
>>
>> --
>> Jens Axboe
>>
> Thanks for your advice. So, you can't accept pass the ioprio by
> set_task_ioprio?

Not sure how many times I'd have to state that, no.

> If only the method of lo_rw_aio() counld you accept? I don't want to
> submit this part of the modifications separately. I just want to know,
> this is ok to you or not?

Inheriting the kiocb ioprio from the request is the right approach, so
yeah that part is fine.

-- 
Jens Axboe