* [PATCH 1/2] block: don't request a module for the none elevator
2026-11-12 3:00 [PATCH 0/2] block: make rewriting the current I/O scheduler cheap again Qiliang Yuan
@ 2026-11-12 3:00 ` Qiliang Yuan
2026-11-12 3:00 ` [PATCH 2/2] block: return early when the elevator doesn't change Qiliang Yuan
1 sibling, 0 replies; 3+ messages in thread
From: Qiliang Yuan @ 2026-11-12 3:00 UTC (permalink / raw)
To: Jens Axboe, Damien Le Moal, Ming Lei, Nilay Shroff, Hannes Reinecke
Cc: linux-block, linux-kernel, Qiliang Yuan
elv_iosched_store() calls elv_iosched_load_module() for the requested
name before anything else, and loads "<name>-iosched" if no elevator
type of that name is registered. "none" never is, so every write of
"none" to queue/scheduler ends up in request_module("none-iosched").
That runs a modprobe helper which can never find the module, and adds
several milliseconds to every switch to "none", including rewrites of
it by udev rules or tuning tools that set the scheduler on each disk.
Return early for "none", like elevator_change() did before the module
loading moved into elv_iosched_load_module().
Writing queue/scheduler of a null_blk disk 20 times takes (median):
operation before (ms) after (ms) change
none -> none 13.09 9.06 -31%
mq-deadline -> none 11.99 7.96 -34%
Fixes: 734e1a8603128 ("block: Prevent deadlocks when switching elevators")
Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
---
block/elevator.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/block/elevator.c b/block/elevator.c
index 66edf1a5768a8..9d9b2e84a1288 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -775,6 +775,9 @@ static void elv_iosched_load_module(const char *elevator_name)
{
struct elevator_type *found;
+ if (!strncmp(elevator_name, "none", 4))
+ return;
+
spin_lock(&elv_list_lock);
found = __elevator_find(elevator_name);
spin_unlock(&elv_list_lock);
--
2.43.0
^ permalink raw reply [flat|nested] 3+ messages in thread* [PATCH 2/2] block: return early when the elevator doesn't change
2026-11-12 3:00 [PATCH 0/2] block: make rewriting the current I/O scheduler cheap again Qiliang Yuan
2026-11-12 3:00 ` [PATCH 1/2] block: don't request a module for the none elevator Qiliang Yuan
@ 2026-11-12 3:00 ` Qiliang Yuan
1 sibling, 0 replies; 3+ messages in thread
From: Qiliang Yuan @ 2026-11-12 3:00 UTC (permalink / raw)
To: Jens Axboe, Damien Le Moal, Ming Lei, Nilay Shroff, Hannes Reinecke
Cc: linux-block, linux-kernel, Qiliang Yuan
Writing the scheduler a queue already uses to queue/scheduler used to
be a no-op. elevator_change() now only checks for an unchanged elevator
after freezing the queue, and doesn't treat "none" on a queue without
an elevator as unchanged at all, so that case also quiesces the queue
in elevator_switch() and resets nr_requests.
Every such write therefore drains in-flight I/O and waits for an RCU
grace period. udev rules and tuning tools that set the scheduler
unconditionally stall I/O on live disks each time they run.
Check under elevator_lock whether anything would change before
allocating scheduler resources and freezing the queue. Elevator changes
of a queue are already serialized by QUEUE_FLAG_NO_ELV_SWITCH,
registration and update_nr_hwq_lock, so drop the check under the freeze.
Writing the current scheduler to queue/scheduler of a null_blk disk 20
times takes (median):
operation before (ms) after (ms) change
none -> none 9.06 0.08 -99%
mq-deadline -> mq-deadline 9.14 0.08 -99%
4k random reads at queue depth 1 for 10 seconds, while "none" is
written to queue/scheduler every 50 ms:
metric before after change
IOPS 342609 395915 +16%
max latency (us) 10839 978 -91%
Fixes: b07a889e83355 ("block: move q->sysfs_lock and queue-freeze under show/store method")
Signed-off-by: Qiliang Yuan <qiliang.yuan@linux.dev>
---
block/elevator.c | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)
diff --git a/block/elevator.c b/block/elevator.c
index 9d9b2e84a1288..979203f518ada 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -654,10 +654,24 @@ static int elevator_change(struct request_queue *q, struct elv_change_ctx *ctx)
{
unsigned int memflags;
struct blk_mq_tag_set *set = q->tag_set;
+ bool unchanged;
int ret = 0;
lockdep_assert_held(&set->update_nr_hwq_lock);
+ /*
+ * Callers serialize elevator changes of this queue, so q->elevator is
+ * stable here. Skip freezing the queue if nothing would change.
+ */
+ mutex_lock(&q->elevator_lock);
+ if (q->elevator)
+ unchanged = elevator_match(q->elevator->type, ctx->name);
+ else
+ unchanged = !strncmp(ctx->name, "none", 4);
+ mutex_unlock(&q->elevator_lock);
+ if (unchanged)
+ return 0;
+
if (strncmp(ctx->name, "none", 4)) {
ret = blk_mq_alloc_sched_res(q, ctx->type, &ctx->res,
set->nr_hw_queues);
@@ -677,8 +691,7 @@ static int elevator_change(struct request_queue *q, struct elv_change_ctx *ctx)
*/
blk_mq_cancel_work_sync(q);
mutex_lock(&q->elevator_lock);
- if (!(q->elevator && elevator_match(q->elevator->type, ctx->name)))
- ret = elevator_switch(q, ctx);
+ ret = elevator_switch(q, ctx);
mutex_unlock(&q->elevator_lock);
blk_mq_unfreeze_queue(q, memflags);
if (!ret)
--
2.43.0
^ permalink raw reply [flat|nested] 3+ messages in thread