block, bfq: remove slow-system class

BFQ computes the duration of weight raising for interactive
applications automatically, using some reference parameters. In
particular, BFQ uses the best durations (see comments in the code for
how these durations have been assessed) for two classes of systems:
slow and fast ones. Examples of slow systems are old phones or systems
using micro HDDs. Fast systems are all the remaining ones. Using these
parameters, BFQ computes the actual duration of the weight raising,
for the system at hand, as a function of the relative speed of the
system w.r.t. the speed of a reference system, belonging to the same
class of systems as the system at hand.

This slow vs fast differentiation proved to be useful in the past, but
happens to have little meaning with current hardware. Even worse, it
does cause problems in virtual systems, where the speed of the system
can vary frequently, and so widely to just confuse the class-detection
mechanism, and, as we have verified experimentally, to cause BFQ to
compute non-sensical weight-raising durations.

This commit addresses this issue by removing the slow class and the
class-detection mechanism.

Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 4 insertions, 10 deletions

diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
index faac509cb35e..0f712e03b035 100644
--- a/block/bfq-iosched.h
+++ b/block/bfq-iosched.h
@@ -399,11 +399,6 @@ struct bfq_io_cq {
 	struct bfq_ttime saved_ttime;
 };
 
-enum bfq_device_speed {
-	BFQ_BFQD_FAST,
-	BFQ_BFQD_SLOW,
-};
-
 /**
  * struct bfq_data - per-device data structure.
  *
@@ -611,12 +606,11 @@ struct bfq_data {
 	/* Max service-rate for a soft real-time queue, in sectors/sec */
 	unsigned int bfq_wr_max_softrt_rate;
 	/*
-	 * Cached value of the product R*T, used for computing the
-	 * maximum duration of weight raising automatically.
+	 * Cached value of the product ref_rate*ref_wr_duration, used
+	 * for computing the maximum duration of weight raising
+	 * automatically.
 	 */
-	u64 RT_prod;
-	/* device-speed class for the low-latency heuristic */
-	enum bfq_device_speed device_speed;
+	u64 rate_dur_prod;
 
 	/* fallback dummy bfqq for extreme OOM conditions */
 	struct bfq_queue oom_bfqq;