block, bfq: introduce Early Queue Merge (v3.5.0, v3.6.0, v3.7.0)

A set of processes may happen  to perform interleaved reads, i.e., the union
of their  requests would give rise  to a sequential read  pattern. There are
two typical  cases: in the first  case, processes read fixed-size  chunks of
data at a fixed distance from each other, while in the second case processes
may read variable-size chunks at  variable distances. The latter case occurs
for  example with  KVM, which  splits the  I/O generated  by the  guest into
multiple chunks,  and lets these chunks  be served by a  pool of cooperating
processes,  iteratively  assigning  the  next  chunk of  I/O  to  the  first
available  process. CFQ  uses actual  queue merging  for the  first type  of
processes, whereas it  uses preemption to get a sequential  read pattern out
of the read requests  performed by the second type of  processes. In the end
it uses  two different  mechanisms to  achieve the  same goal:  boosting the
throughput with interleaved I/O.

This patch introduces Early Queue Merge  (EQM), a unified mechanism to get a
sequential  read pattern  with both  types of  processes. The  main idea  is
checking newly arrived requests against the next request of the active queue
both in case of actual request insert and in case of request merge. By doing
so, both the types of processes can be handled by just merging their queues.
EQM is  then simpler and  more compact than the  pair of mechanisms  used in
CFQ.

Finally, EQM  also preserves the  typical low-latency properties of  BFQ, by
properly restoring the weight-raising state of  a queue when it gets back to
a non-merged state.

Signed-off-by: Mauro Andreolini <mauro.andreolini@unimore.it>
Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
Reviewed-by: Paolo Valente <paolo.valente@unimore.it>