Re: [PATCH] blk-ioprio: Introduce promote-to-rt policy

Bart Van Assche
2023-02-09T19:09:33Z

On 2/9/23 00:56, Jan Kara wrote:
> On Wed 08-02-23 09:53:41, Bart Van Assche wrote:
>> The test results I shared some time ago show that IOPRIO_CLASS_NONE was the
>> default I/O priority two years ago (see also https://lore.kernel.org/linux-block/20210927220328.1410161-5-bvanassche@acm.org/).
>> The none-to-rt policy increases the priority of bio's that have not been
>> assigned an I/O priority to RT. Does this answer your question?
>
> Not quite. I know that historically we didn't set bio I/O priority in some
> paths (but we did set it in other paths such as some (but not all) direct
> IO implementations). But that was exactly a mess because how none-to-rt
> actually behaved depended on the exact details of the kernel internal IO
> path. So my question is: Was none-to-rt actually just a misnomer and the
> intended behavior was "always override to RT"? Or what was exactly the
> expectation around when IO priority is not set and should be overridden?
>
> How should it interact with AIO submissions with IOCB_FLAG_IOPRIO? How
> should it interact with task having its IO priority modified with
> ioprio_set(2)? And what if task has its normal scheduling priority modified
> but that translates into different IO priority (which happens in
> __get_task_ioprio())?
>
> So I think that none-to-rt is just poorly defined and if we can just get
> rid of it (or redefine to promote-to-rt), that would be good. But maybe I'm
> missing some intended usecase...

Hi Jan,

We have no plans to use the ioprio_set() system call since it only
affects foreground I/O and not page cache writeback.

While Android supports io_uring, there are no plans to support libaio in
the Android C library (Bionic).

Regarding __get_task_ioprio(), I haven't found any code in that function
that derives an I/O priority from the scheduling priority. Did I perhaps
overlook something?

Until recently "none-to-rt" meant "if no I/O priority has been assigned
to a task, use IOPRIO_CLASS_RT". Promoting the I/O priority to
IOPRIO_CLASS_RT works for us. I'm fine with changing the meaning of
"none-to-rt" into promoting the I/O priority class to RT. Introducing
"promote-to-rt" as a synonym of "none-to-rt" is also fine with me.

Thanks,

Bart.