Re: [PATCH v3 1/2] block: Add iocontext priority to request

[Jens Axboe <axboe@kernel.dk>]

On 10/12/2016 11:58 AM, Adam Manzanares wrote:
> The 10/12/2016 08:49, Jens Axboe wrote:
>> On 10/11/2016 02:40 PM, Adam Manzanares wrote:
>>> Patch adds an association between iocontext ioprio and the ioprio of
>>> a request. This feature is only enabled if a queue flag is set to
>>> indicate that requests should have ioprio associated with them. The
>>> queue flag is exposed as the req_prio queue sysfs entry.
>>
>> Honestly, I don't get this patchset. For the normal file system path, we
>> inherit the iocontext priority into the bio. That in turns gets
>> inherited by the request. Why is this any different?
>>
> I was hoping this was true before I started looking into this, but the
> iocontext priority is not set on the bio. I did look into setting the
> iocontext priority on the bio, and this would have do be done close
> in the call stack to init_request_from_bio so I chose to set it here.
>
> If I missed where this occurs I would appreciate it if you pointed me
> to the code where the bio gets the iopriority from the iocontext.

It currently happens in CFQ, since that is what deals with priorities.
But we should just make that the case, generically. The rule is that if
priority is set in a bio, that is what we'll use. But if not, we'll
use what the application has set in general, and that is available in
the io context.

>> It'd be much cleaner to just have 'rq' inherit the IO priority from the
>> io context when the 'rq' is allocated, and ensuring that we inherit and
>> override this if the bio has one set instead. It should already work
>> like that.
>
> I looked into the request allocation code and the only place I see where
> the iocontext is associated with the request is through a icq. Looking at
> the documentation of the icq it states that the icq_size of the elevator
> has to be set in order for block core to manage this. The only scheduler
> using this currently is the cfq scheduler and I think prioritized requests
> should be independent of the scheduler used.

Agree, see above, it should just happen generically.

>> And in no way should we add some sysfs file to control this, that is
>> nuts.
>
> My concern is that we will now be exposing priority information to lower
> layers and if there happens to be code that uses the priority now it will
> actually make an impact. My example being the fusion mptsas driver.
>
> The second issue I foresee is that lower level drivers will need a sysfs
> file to control whether or not we send prioritized commands to the device.
> We are wary of sending prioritized commands by default when we are unsure
> of how the device will respond to these prioritized commands.
>
> The reason I propose a sysfs file in the queue is that it solves these two
> issues at the same time.
>
> I would appreciate it if you could suggest an alternative fix for
> these issues or an explanation of why these concerns are not valid.

At that point it should be a driver knob, it's not something that should
be part of the generic block layer.

-- 
Jens Axboe