Re: [PATCH 3/3] block/mq-deadline: Disable I/O prioritization in certain cases

[Damien Le Moal <dlemoal@kernel.org>]

On 12/13/23 10:02, Bart Van Assche wrote:
> On 12/12/23 13:52, Damien Le Moal wrote:
>> Trying to solve this issue in mq-deadline would require keeping track of the io
>> priority used for a write request that is issued to a zone and use that same
>> priority for all following write requests for the same zone until there are no
>> writes pending for that zone. Otherwise, you will get the priority inversion
>> causing the reordering.
>>
>> But I think that doing all this without also causing priority inversion for the
>> user, i.e. a high priority write request ends up waiting for a low priority one,
>> will be challenging, to say the least.
> 
> Hi Damien,
> 
> How about the following algorithm?
> - If a zoned write refers to the start of a zone or no other writes for
>    the same zone occur in the RB-tree, use the I/O priority of the zoned
>    write.
> - If another write for the same zone occurs in the RB-tree, use the I/O
>    priority from that other write.
> 
> While this algorithm does not guarantee that all zoned writes for a 
> single zone have the same I/O priority, it guarantees that the 
> mq-deadline I/O scheduler won't submit zoned writes in the wrong order 
> because of their I/O priority.

I guess this should work.

> 
> Thanks,
> 
> Bart.
> 

-- 
Damien Le Moal
Western Digital Research