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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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.

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.

Thanks,
Paolo

Paolo
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