> Il giorno 17 gen 2017, alle ore 03:47, Jens Axboe <axboe@xxxxxx> ha scritto:
>
> On 12/22/2016 02:59 AM, Paolo Valente wrote:
>>
>>> Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe <axboe@xxxxxx> ha scritto:
>>>
>>> This adds a set of hooks that intercepts the blk-mq path of
>>> allocating/inserting/issuing/completing requests, allowing
>>> us to develop a scheduler within that framework.
>>>
>>> We reuse the existing elevator scheduler API on the registration
>>> side, but augment that with the scheduler flagging support for
>>> the blk-mq interfce, and with a separate set of ops hooks for MQ
>>> devices.
>>>
>>> Schedulers can opt in to using shadow requests. Shadow requests
>>> are internal requests that the scheduler uses for for the allocate
>>> and insert part, which are then mapped to a real driver request
>>> at dispatch time. This is needed to separate the device queue depth
>>> from the pool of requests that the scheduler has to work with.
>>>
>>> Signed-off-by: Jens Axboe <axboe@xxxxxx>
>>>
>> ...
>>
>>> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
>>> new file mode 100644
>>> index 000000000000..b7e1839d4785
>>> --- /dev/null
>>> +++ b/block/blk-mq-sched.c
>>
>>> ...
>>> +static inline bool
>>> +blk_mq_sched_allow_merge(struct request_queue *q, struct request *rq,
>>> + struct bio *bio)
>>> +{
>>> + struct elevator_queue *e = q->elevator;
>>> +
>>> + if (e && e->type->ops.mq.allow_merge)
>>> + return e->type->ops.mq.allow_merge(q, rq, bio);
>>> +
>>> + return true;
>>> +}
>>> +
>>
>> Something does not seem to add up here:
>> e->type->ops.mq.allow_merge may be called only in
>> blk_mq_sched_allow_merge, which, in its turn, may be called only in
>> blk_mq_attempt_merge, which, finally, may be called only in
>> blk_mq_merge_queue_io. Yet the latter may be called only if there is
>> no elevator (line 1399 and 1507 in blk-mq.c).
>>
>> Therefore, e->type->ops.mq.allow_merge can never be called, both if
>> there is and if there is not an elevator. Be patient if I'm missing
>> something huge, but I thought it was worth reporting this.
>
> I went through the current branch, and it seems mostly fine. There was
> a double call to allow_merge() that I killed in the plug path, and one
> set missing in blk_mq_sched_try_merge(). The rest looks OK.
>
Yes, I missed a path, sorry. I'm happy that at least your check has
not been a waste of time for other reasons.
Thanks,
Paolo
> --
> Jens Axboe
>
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