Il giorno 22 dic 2016, alle ore 10:59, Paolo Valente =

[off-list ref] ha scritto:

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quoted

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Il giorno 17 dic 2016, alle ore 01:12, Jens Axboe [off-list ref] ha =

scritto:

quoted

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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.
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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.
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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.
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Signed-off-by: Jens Axboe <axboe@fb.com>
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...
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quoted

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

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quoted

...
+static inline bool
+blk_mq_sched_allow_merge(struct request_queue *q, struct request =

*rq,

quoted

+			 struct bio *bio)
+{
+	struct elevator_queue *e =3D q->elevator;
+
+	if (e && e->type->ops.mq.allow_merge)
+		return e->type->ops.mq.allow_merge(q, rq, bio);
+
+	return true;
+}
+

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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).
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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.
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Jens,
I forgot to add that I'm willing (and would be happy) to propose a fix
to this, and possibly the other problems too, on my own.  Just, I'm
not yet expert enough to do it with having first received some
feedback or instructions from you.  In this specific case, I don't
even know yet whether this is really a bug.

Thanks, and merry Christmas if we don't get in touch before,
Paolo

Paolo
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