block, bfq: move debug blkio stats behind CONFIG_DEBUG_BLK_CGROUP

BFQ currently creates, and updates, its own instance of the whole
set of blkio statistics that cfq creates. Yet, from the comments
of Tejun Heo in [1], it turned out that most of these statistics
are meant/useful only for debugging. This commit makes BFQ create
the latter, debugging statistics only if the option
CONFIG_DEBUG_BLK_CGROUP is set.

By doing so, this commit also enables BFQ to enjoy a high perfomance
boost. The reason is that, if CONFIG_DEBUG_BLK_CGROUP is not set, then
BFQ has to update far fewer statistics, and, in particular, not the
heaviest to update.  To give an idea of the benefits, if
CONFIG_DEBUG_BLK_CGROUP is not set, then, on an Intel i7-4850HQ, and
with 8 threads doing random I/O in parallel on null_blk (configured
with 0 latency), the throughput of BFQ grows from 310 to 400 KIOPS
(+30%). We have measured similar or even much higher boosts with other
CPUs: e.g., +45% with an ARM CortexTM-A53 Octa-core. Our results have
been obtained and can be reproduced very easily with the script in [1].

[1] https://www.spinics.net/lists/linux-block/msg18943.html

Suggested-by: Tejun Heo <tj@kernel.org>
Suggested-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Lee Tibbert <lee.tibbert@gmail.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Luca Miccio <lucmiccio@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/block/bfq-iosched.txt

@@ -20,12 +20,22 @@ for that device, by setting low_latency to 0. See Section 3 for
 details on how to configure BFQ for the desired tradeoff between
 latency and throughput, or on how to maximize throughput.
 
-BFQ has a non-null overhead, which limits the maximum IOPS that the
-CPU can process for a device scheduled with BFQ. To give an idea of
-the limits on slow or average CPUs, here are BFQ limits for three
-different CPUs, on, respectively, an average laptop, an old desktop,
-and a cheap embedded system, in case full hierarchical support is
-enabled (i.e., CONFIG_BFQ_GROUP_IOSCHED is set):
+BFQ has a non-null overhead, which limits the maximum IOPS that a CPU
+can process for a device scheduled with BFQ. To give an idea of the
+limits on slow or average CPUs, here are, first, the limits of BFQ for
+three different CPUs, on, respectively, an average laptop, an old
+desktop, and a cheap embedded system, in case full hierarchical
+support is enabled (i.e., CONFIG_BFQ_GROUP_IOSCHED is set), but
+CONFIG_DEBUG_BLK_CGROUP is not set (Section 4-2):
+- Intel i7-4850HQ: 400 KIOPS
+- AMD A8-3850: 250 KIOPS
+- ARM CortexTM-A53 Octa-core: 80 KIOPS
+
+If CONFIG_DEBUG_BLK_CGROUP is set (and of course full hierarchical
+support is enabled), then the sustainable throughput with BFQ
+decreases, because all blkio.bfq* statistics are created and updated
+(Section 4-2). For BFQ, this leads to the following maximum
+sustainable throughputs, on the same systems as above:
 - Intel i7-4850HQ: 310 KIOPS
 - AMD A8-3850: 200 KIOPS
 - ARM CortexTM-A53 Octa-core: 56 KIOPS
@@ -505,6 +515,22 @@ BFQ-specific files is "blkio.bfq." or "io.bfq." For example, the group
 parameter to set the weight of a group with BFQ is blkio.bfq.weight
 or io.bfq.weight.
 
+As for cgroups-v1 (blkio controller), the exact set of stat files
+created, and kept up-to-date by bfq, depends on whether
+CONFIG_DEBUG_BLK_CGROUP is set. If it is set, then bfq creates all
+the stat files documented in
+Documentation/cgroup-v1/blkio-controller.txt. If, instead,
+CONFIG_DEBUG_BLK_CGROUP is not set, then bfq creates only the files
+blkio.bfq.io_service_bytes
+blkio.bfq.io_service_bytes_recursive
+blkio.bfq.io_serviced
+blkio.bfq.io_serviced_recursive
+
+The value of CONFIG_DEBUG_BLK_CGROUP greatly influences the maximum
+throughput sustainable with bfq, because updating the blkio.bfq.*
+stats is rather costly, especially for some of the stats enabled by
+CONFIG_DEBUG_BLK_CGROUP.
+
 Parameters to set
 -----------------