[PATCH v2 25/83] block: rnull: add no_sched module parameter and configfs attribute

[Andreas Hindborg]

Add support for disabling the default IO scheduler by adding:
- no_sched module parameter to control scheduler selection at device
  creation.
- no_sched configfs attribute (ID 11) for runtime configuration.
- Use of NO_DEFAULT_SCHEDULER flag when no_sched is enabled.

This allows bypassing the default 'mq-deadline' scheduler and using 'none'
instead, which can improve performance for certain workloads. The flag
selection logic is updated to use compound assignment operators for better
readability.

Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
---
 drivers/block/rnull/configfs.rs |  6 ++++++
 drivers/block/rnull/rnull.rs    | 25 ++++++++++++++++++-------
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/drivers/block/rnull/configfs.rs b/drivers/block/rnull/configfs.rs
index e47399cd45a4..d9aead646ae0 100644
--- a/drivers/block/rnull/configfs.rs
+++ b/drivers/block/rnull/configfs.rs
@@ -94,6 +94,7 @@ fn make_group(
                 use_per_node_hctx: 8,
                 home_node: 9,
                 discard: 10,
+                no_sched:11,
             ],
         };
 
@@ -115,6 +116,7 @@ fn make_group(
                     submit_queues: 1,
                     home_node: bindings::NUMA_NO_NODE,
                     discard: false,
+                    no_sched: false,
                 }),
             }),
             core::iter::empty(),
@@ -183,6 +185,7 @@ struct DeviceConfigInner {
     submit_queues: u32,
     home_node: i32,
     discard: bool,
+    no_sched: bool,
 }
 
 #[vtable]
@@ -217,6 +220,7 @@ fn store(this: &DeviceConfig, page: &[u8]) -> Result {
                 submit_queues: guard.submit_queues,
                 home_node: guard.home_node,
                 discard: guard.discard,
+                no_sched: guard.no_sched,
             })?);
             guard.powered = true;
         } else if guard.powered && !power_op {
@@ -322,3 +326,5 @@ fn store(this: &DeviceConfig, page: &[u8]) -> Result {
         Ok(())
     })
 );
+
+configfs_simple_bool_field!(DeviceConfig, 11, no_sched);
diff --git a/drivers/block/rnull/rnull.rs b/drivers/block/rnull/rnull.rs
index bdc05b3f6072..cb5b642f68e5 100644
--- a/drivers/block/rnull/rnull.rs
+++ b/drivers/block/rnull/rnull.rs
@@ -30,8 +30,8 @@
     new_mutex,
     new_xarray,
     page::{
-        SafePage, //
-        PAGE_SIZE,
+        SafePage,
+        PAGE_SIZE, //
     },
     pr_info,
     prelude::*,
@@ -110,6 +110,10 @@
             description:
                 "Support discard operations (requires memory-backed null_blk device).",
         },
+        no_sched: bool {
+            default: false,
+            description: "No IO scheduler",
+        },
     },
 }
 
@@ -148,6 +152,7 @@ fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
                     submit_queues,
                     home_node: module_parameters::home_node.value(),
                     discard: module_parameters::discard.value(),
+                    no_sched: module_parameters::no_sched.value(),
                 })?;
                 disks.push(disk, GFP_KERNEL)?;
             }
@@ -173,6 +178,7 @@ struct NullBlkOptions<'a> {
     submit_queues: u32,
     home_node: i32,
     discard: bool,
+    no_sched: bool,
 }
 struct NullBlkDevice;
 
@@ -189,13 +195,18 @@ fn new(options: NullBlkOptions<'_>) -> Result<GenDisk<Self>> {
             submit_queues,
             home_node,
             discard,
+            no_sched,
         } = options;
 
-        let flags = if memory_backed {
-            mq::tag_set::Flag::Blocking.into()
-        } else {
-            mq::tag_set::Flags::default()
-        };
+        let mut flags = mq::tag_set::Flags::default();
+
+        if memory_backed {
+            flags |= mq::tag_set::Flag::Blocking;
+        }
+
+        if no_sched {
+            flags |= mq::tag_set::Flag::NoDefaultScheduler;
+        }
 
         if home_node > kernel::numa::num_online_nodes().try_into()? {
             return Err(code::EINVAL);

-- 
2.51.2