Re: [PATCH 1/3] block: move io_context creation into where it's needed

[Jens Axboe <axboe@kernel.dk>]

On 11/23/21 9:46 AM, Jens Axboe wrote:
> On 11/23/21 9:39 AM, Christoph Hellwig wrote:
>>> +	/* create task io_context, if we don't have one already */
>>> +	if (unlikely(!current->io_context))
>>> +		create_task_io_context(current, GFP_ATOMIC, rq->q->node);
>>
>> Wouldn't it be nicer to have an interface that hides the check
>> and the somewhat pointless current argument?  And name that with a
>> blk_ prefix?
> 
> Yeah, we can do that.
> 
>>> +
>>> +	blk_mq_sched_assign_ioc(rq);
>>
>> But thinking about this a little more:
>>
>> struct io_context has two uses, one is the unsigned short ioprio,
>> and the other is the whole bfq mess.
>>
>> Can't we just split the ioprio out (we'll find a hole somewhere
>> in task_struct) and then just allocate the io_context in
>> blk_mq_sched_assign_ioc, which would also avoid the pointless
>> ioc_lookup_icq on a newly allocated io_context.  I'd also really
>> expect blk_mq_sched_assign_ioc to be implemented in blk-ioc.c.
> 
> It would be nice to decouple ioprio from the io_context, and just
> leave the io_context for BFQ essentially.
> 
> I'll give it a shot.

Actually may not be that trivial - if a thread is cloned with CLONE_IO,
then it shares the io_context, and hence also the io priority. That will
auto-propagate if one linked task changes it, and putting it in the
task_struct would break that.

So I don't think we can do that - which is a shame, as it would be a
nice cleanup.

-- 
Jens Axboe