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Subject[PATCH v1] block, bfq: set default slice_idle to zero for SSDs
With default 8ms as a slice idle time, we seen few time bounded
applications(sensors) on v4.19 kernel are getting timedout during
multimedia tests (audio, video playbacks etc) with Reboots and
leading to crash. The timeout configured for these applications
(sensors) are 20sec.

In crash dumps, we seen few synchronous requests from sensors/other
applications were in their bfq_queues for more than 12-20sec.

Idling due to anticipation of future near-by IO requests and wait on
completion of submitted requests, will effect in choosing the next
bfq-queue and its scheduling. There by it effecting some time bounded

After making the slice idle to zero, we didn't seen any crash during
our 72hrs of testing and also it increases the IO throughput.

Following FIO benchmark results were taken on a local SSD run:

RandomReads that were taken on v4.19 kernel:

Idling iops avg-lat(us) stddev bw
On 4136 1189.07 17221.65 16.9MB/s
Off 7246 670.11 1054.76 29.7MB/s

fio --name=temp --size=5G --time_based --ioengine=sync \
--randrepeat=0 --direct=1 --invalidate=1 --verify=0 \
--verify_fatal=0 --rw=randread --blocksize=4k \
--group_reporting=1 --directory=/data --runtime=10 \
--iodepth=64 --numjobs=5

Following code changes were made based on [1],[2] and [3].

[2] Commit 41c0126b3f22 ("block: Make CFQ default to IOPS mode on
[3] Commit 0bb979472a74 ("cfq-iosched: fix the setting of IOPS mode on

Signed-off-by: Pradeep P V K <>
Documentation/block/bfq-iosched.rst | 7 ++++---
block/bfq-iosched.c | 13 +++++++++++++
block/elevator.c | 2 ++
include/linux/elevator.h | 1 +
4 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/Documentation/block/bfq-iosched.rst b/Documentation/block/bfq-iosched.rst
index 0d237d4..244f4ca 100644
--- a/Documentation/block/bfq-iosched.rst
+++ b/Documentation/block/bfq-iosched.rst
@@ -329,9 +329,10 @@ slice_idle

This parameter specifies how long BFQ should idle for next I/O
request, when certain sync BFQ queues become empty. By default
-slice_idle is a non-zero value. Idling has a double purpose: boosting
-throughput and making sure that the desired throughput distribution is
-respected (see the description of how BFQ works, and, if needed, the
+slice_idle is a non-zero value for rotational devices.
+Idling has a double purpose: boosting throughput and making
+sure that the desired throughput distribution is respected
+(see the description of how BFQ works, and, if needed, the
papers referred there).

As for throughput, idling can be very helpful on highly seeky media
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 0319d63..9c994d1 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -6514,6 +6514,18 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
return -ENOMEM;

+static void bfq_registered_queue(struct request_queue *q)
+ struct elevator_queue *e = q->elevator;
+ struct bfq_data *bfqd = e->elevator_data;
+ /*
+ * Default to IOPS mode with no idling for SSDs
+ */
+ if (blk_queue_nonrot(q))
+ bfqd->bfq_slice_idle = 0;
static void bfq_slab_kill(void)
@@ -6761,6 +6773,7 @@ static ssize_t bfq_low_latency_store(struct elevator_queue *e,
.init_hctx = bfq_init_hctx,
.init_sched = bfq_init_queue,
.exit_sched = bfq_exit_queue,
+ .elevator_registered_fn = bfq_registered_queue,

.icq_size = sizeof(struct bfq_io_cq),
diff --git a/block/elevator.c b/block/elevator.c
index 076ba73..b882d25 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -504,6 +504,8 @@ int elv_register_queue(struct request_queue *q, bool uevent)
kobject_uevent(&e->kobj, KOBJ_ADD);

e->registered = 1;
+ if (e->type->ops.elevator_registered_fn)
+ e->type->ops.elevator_registered_fn(q);
return error;
diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index 901bda3..23dcc35 100644
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -50,6 +50,7 @@ struct elevator_mq_ops {
struct request *(*next_request)(struct request_queue *, struct request *);
void (*init_icq)(struct io_cq *);
void (*exit_icq)(struct io_cq *);
+ void (*elevator_registered_fn)(struct request_queue *q);

#define ELV_NAME_MAX (16)
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