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

[Jens Axboe <axboe@kernel.dk>]

On 5/23/24 12:04 AM, yunlong xing wrote:
> Bart Van Assche <bvanassche@acm.org> ?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