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

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Date: Wed, 22 May 2024 11:12:16 -0700
From: Bart Van Assche <bvanassche@....org>
To: Jens Axboe <axboe@...nel.dk>, Yunlong Xing <yunlong.xing@...soc.com>,
 yunlongxing23@...il.com, niuzhiguo84@...il.com, Hao_hao.Wang@...soc.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] loop: inherit the ioprio in loop woker thread

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?

Thanks,

Bart.