[PATCH 22/79] block: rnull: add no_sched module parameter and configfs attribute

[Andreas Hindborg <a.hindborg@kernel.org>]

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@kernel.org>
---
 drivers/block/rnull/configfs.rs | 31 +++++++++++++++++++++++++++++++
 drivers/block/rnull/rnull.rs    | 25 ++++++++++++++++++-------
 2 files changed, 49 insertions(+), 7 deletions(-)

diff --git a/drivers/block/rnull/configfs.rs b/drivers/block/rnull/configfs.rs
index 4ec9be440105d..18e32a87673aa 100644
--- a/drivers/block/rnull/configfs.rs
+++ b/drivers/block/rnull/configfs.rs
@@ -95,6 +95,7 @@ fn make_group(
                 use_per_node_hctx: 8,
                 home_node: 9,
                 discard: 10,
+                no_sched:11,
             ],
         };
 
@@ -116,6 +117,7 @@ fn make_group(
                     submit_queues: 1,
                     home_node: bindings::NUMA_NO_NODE,
                     discard: false,
+                    no_sched: false,
                 }),
             }),
             core::iter::empty(),
@@ -174,6 +176,7 @@ struct DeviceConfigInner {
     submit_queues: u32,
     home_node: i32,
     discard: bool,
+    no_sched: bool,
 }
 
 #[vtable]
@@ -208,6 +211,7 @@ 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 {
@@ -336,3 +340,30 @@ fn store(this: &DeviceConfig, page: &[u8]) -> Result {
         Ok(())
     })
 );
+
+#[vtable]
+impl configfs::AttributeOperations<11> for DeviceConfig {
+    type Data = DeviceConfig;
+
+    fn show(this: &DeviceConfig, page: &mut [u8; PAGE_SIZE]) -> Result<usize> {
+        let mut writer = kernel::str::Formatter::new(page);
+
+        if this.data.lock().no_sched {
+            writer.write_str("1\n")?;
+        } else {
+            writer.write_str("0\n")?;
+        }
+
+        Ok(writer.bytes_written())
+    }
+
+    fn store(this: &DeviceConfig, page: &[u8]) -> Result {
+        if this.data.lock().powered {
+            return Err(EBUSY);
+        }
+
+        this.data.lock().no_sched = kstrtobool_bytes(page)?;
+
+        Ok(())
+    }
+}
diff --git a/drivers/block/rnull/rnull.rs b/drivers/block/rnull/rnull.rs
index d1fb5fbc0c0be..9a301b06fc5e0 100644
--- a/drivers/block/rnull/rnull.rs
+++ b/drivers/block/rnull/rnull.rs
@@ -29,8 +29,8 @@
     new_mutex,
     new_xarray,
     page::{
-        SafePage, //
-        PAGE_SIZE,
+        SafePage,
+        PAGE_SIZE, //
     },
     pr_info,
     prelude::*,
@@ -112,6 +112,10 @@
             description:
             "Support discard operations (requires memory-backed null_blk device). Default: false",
         },
+        no_sched: u8 {
+            default: 0,
+            description: "No IO scheduler",
+        },
     },
 }
 
@@ -150,6 +154,7 @@ fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
                     submit_queues,
                     home_node: *module_parameters::home_node.value(),
                     discard: *module_parameters::discard.value() != 0,
+                    no_sched: *module_parameters::no_sched.value() != 0,
                 })?;
                 disks.push(disk, GFP_KERNEL)?;
             }
@@ -175,6 +180,7 @@ struct NullBlkOptions<'a> {
     submit_queues: u32,
     home_node: i32,
     discard: bool,
+    no_sched: bool,
 }
 struct NullBlkDevice;
 
@@ -191,13 +197,18 @@ 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::num_online_nodes().try_into()? {
             return Err(code::EINVAL);

-- 
2.51.2