Date:   Tue, 17 Jan 2017 11:13:18 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Linux-Kernal <linux-kernel@...r.kernel.org>, osandov@...com
Subject: Re: [PATCH 6/8] blk-mq-sched: add framework for MQ capable IO schedulers


> Il giorno 17 gen 2017, alle ore 03:47, Jens Axboe <axboe@...com> ha scritto:
> 
> On 12/22/2016 04:13 AM, Paolo Valente wrote:
>> 
>>> Il giorno 22 dic 2016, alle ore 10:59, Paolo Valente <paolo.valente@...aro.org> ha scritto:
>>> 
>>>> 
>>>> Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe <axboe@...com> 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@...com>
>>>> 
>>> ...
>>> 
>>>> 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.
>>> 
>> 
>> Just another detail: if e->type->ops.mq.allow_merge does get invoked
>> from the above path, then it is invoked of course without the
>> scheduler lock held.  In contrast, if this function gets invoked
>> from dd_bio_merge, then the scheduler lock is held.
> 
> But the scheduler controls that itself. So it'd be perfectly fine to
> have a locked and unlocked variant. The way that's typically done is to
> have function() grabbing the lock, and __function() is invoked with the
> lock held.
> 
>> To handle this opposite alternatives, I don't know whether checking if
>> the lock is held (and possibly taking it) from inside
>> e->type->ops.mq.allow_merge is a good solution.  In any case, before
>> possibly trying it, I will wait for some feedback on the main problem,
>> i.e., on the fact that e->type->ops.mq.allow_merge
>> seems unreachable in the above path.
> 
> Checking if a lock is held is NEVER a good idea, as it leads to both bad
> and incorrect code. If you just check if a lock is held when being
> called, you don't necessarily know if it was the caller that grabbed it
> or it just happens to be held by someone else for unrelated reasons.
> 
> 

Thanks a lot for this and the above explanations.  Unfortunately, I
still see the problem.  To hopefully make you waste less time, I have
reported the problematic paths explicitly, so that you can quickly
point me to my mistake.

The problem is caused by the existence of at least the following two
alternative paths to e->type->ops.mq.allow_merge.

1.  In mq-deadline.c (line 374): spin_lock(&dd->lock);
blk_mq_sched_try_merge -> elv_merge -> elv_bio_merge_ok ->
elv_iosched_allow_bio_merge -> e->type->ops.mq.allow_merge

2. In blk-core.c (line 1660): spin_lock_irq(q->queue_lock);
elv_merge -> elv_bio_merge_ok ->
elv_iosched_allow_bio_merge -> e->type->ops.mq.allow_merge

In the first path, the scheduler lock is held, while in the second
path, it is not.  This does not cause problems with mq-deadline,
because the latter just has no allow_merge function.  Yet it does
cause problems with the allow_merge implementation of bfq.  There was
no issue in blk, as only the global queue lock was used.

Where am I wrong?

Thanks,
Paolo


> -- 
> Jens Axboe
> 

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