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SubjectRe: [PATCH 7/8] mq-deadline: add blk-mq adaptation of the deadline IO scheduler
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On 12/22/2016 09:07 AM, Paolo Valente wrote:
>
>> Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe <axboe@fb.com> ha scritto:
>>
>> This is basically identical to deadline-iosched, except it registers
>> as a MQ capable scheduler. This is still a single queue design.
>>
>> Signed-off-by: Jens Axboe <axboe@fb.com>
>> ---
>
> ...
>
>> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
>> new file mode 100644
>> index 000000000000..3cb9de21ab21
>> --- /dev/null
>> +++ b/block/mq-deadline.c
>> ...
>> +/*
>> + * remove rq from rbtree and fifo.
>> + */
>> +static void deadline_remove_request(struct request_queue *q, struct request *rq)
>> +{
>> + struct deadline_data *dd = q->elevator->elevator_data;
>> +
>> + list_del_init(&rq->queuelist);
>> +
>> + /*
>> + * We might not be on the rbtree, if we are doing an insert merge
>> + */
>> + if (!RB_EMPTY_NODE(&rq->rb_node))
>> + deadline_del_rq_rb(dd, rq);
>> +
>
> I've been scratching my head on the last three instructions, but at no
> avail. If I understand correctly, the
> list_del_init(&rq->queue list);
> removes rq from the fifo list. But, if so, I don't understand how it
> could be possible that rq has not been added to the rb_tree too.
>
> Another interpretation that I tried is that the above three lines
> handle correctly the following case where rq has not been inserted at
> all into deadline fifo queue and rb tree: when dd_insert_request was
> executed for rq, blk_mq_sched_try_insert_merge succeeded. Yet, the
> list_del_init(&rq->queue list);
> does not seem to make sense.
>
> Could you please shed some light on this for me?

I think you are correct, we don't need to touch ->queuelist for the case
where RB_EMPTY_NODE() is true. Minor detail, the list is already empty,
so it does no harm.

--
Jens Axboe

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