sched/cache: Keep nr_pref_llc_running in the runnable domain, to fix LLC mis-scheduling buggrafted

alb_break_llc() decides whether to break LLC preference during active
load balance. It does so by testing that every runnable fair task on the
source rq prefers its LLC:

	env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable

But the two counters cover different sets. nr_pref_llc_running is updated
in account_llc_enqueue()/account_llc_dequeue(), next to cfs_rq->nr_queued,
so it follows queued tasks. h_nr_runnable is updated in set_delayed()/
clear_delayed() and drops delay-dequeued tasks.

So under DELAY_DEQUEUE, a preferring task that goes to sleep stays counted
in nr_pref_llc_running while h_nr_runnable falls. The equality then breaks,
alb_break_llc() returns false, and active balance is free to pull a task
off its preferred LLC. Active balance only moves runnable tasks, and this
is the only LLC check it consults: once the stopper runs, LBF_ACTIVE_LB
skips the per-task test in can_migrate_task(). The runnable set is the one
we want.

Fix it on the counter side. A task should be counted in
nr_pref_llc_running exactly while it is both queued on its preferred LLC
(pref_llc_queued) and runnable (!sched_delayed). Define that membership
once in task_pref_llc_runnable(), and adjust the counter only through
pref_llc_running_inc()/pref_llc_running_dec() from the four sites that
change either input: account_llc_enqueue(), account_llc_dequeue(),
set_delayed() and clear_delayed(). Gating every update on the same
predicate keeps the delay, wake and dequeue paths from double-counting
or underflowing; see the comments at those sites for the ordering.

nr_llc_running and sd->llc_counts are not touched and stay on queued
semantics.

Fixes: 714059f79ff0 ("sched/cache: Handle moving single tasks to/from their preferred LLC")
Closes: https://lore.kernel.org/lkml/20260827135000.735138-1-zhanxusheng@xiaomi.com/
Reported-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Suggested-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kayra Cizmeci <kayracizmeci@gmail.com>
Cc: <stable@kernel.org> # v7.2.x
Link: https://patch.msgid.link/06af61afedac32e6477f57feb4d658f6c411c3af.1790035273.git.tim.c.chen@linux.intel.com

1 files changed, 100 insertions, 0 deletions

diff --git a/drivers/block/rnull/rnull.rs b/drivers/block/rnull/rnull.rs
new file mode 100644
index 000000000..0ca8715fe
--- /dev/null
+++ b/drivers/block/rnull/rnull.rs
@@ -0,0 +1,100 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! This is a Rust implementation of the C null block driver.
+
+mod configfs;
+
+use configfs::IRQMode;
+use kernel::{
+    block::{
+        self,
+        mq::{
+            self,
+            gen_disk::{self, GenDisk},
+            Operations, TagSet,
+        },
+    },
+    prelude::*,
+    sync::{aref::ARef, Arc},
+};
+
+module! {
+    type: NullBlkModule,
+    name: "rnull_mod",
+    authors: ["Andreas Hindborg"],
+    description: "Rust implementation of the C null block driver",
+    license: "GPL v2",
+}
+
+#[pin_data]
+struct NullBlkModule {
+    #[pin]
+    configfs_subsystem: kernel::configfs::Subsystem<configfs::Config>,
+}
+
+impl kernel::InPlaceModule for NullBlkModule {
+    fn init(_module: &'static ThisModule) -> impl PinInit<Self, Error> {
+        pr_info!("Rust null_blk loaded\n");
+
+        try_pin_init!(Self {
+            configfs_subsystem <- configfs::subsystem(),
+        })
+    }
+}
+
+struct NullBlkDevice;
+
+impl NullBlkDevice {
+    fn new(
+        name: &CStr,
+        block_size: u32,
+        rotational: bool,
+        capacity_mib: u64,
+        irq_mode: IRQMode,
+    ) -> Result<GenDisk<Self>> {
+        let tagset = Arc::pin_init(TagSet::new(1, 256, 1), GFP_KERNEL)?;
+
+        let queue_data = Box::new(QueueData { irq_mode }, GFP_KERNEL)?;
+
+        gen_disk::GenDiskBuilder::new()
+            .capacity_sectors(capacity_mib << (20 - block::SECTOR_SHIFT))
+            .logical_block_size(block_size)?
+            .physical_block_size(block_size)?
+            .rotational(rotational)
+            .build(fmt!("{}", name.to_str()?), tagset, queue_data)
+    }
+}
+
+struct QueueData {
+    irq_mode: IRQMode,
+}
+
+#[vtable]
+impl Operations for NullBlkDevice {
+    type QueueData = KBox<QueueData>;
+
+    #[inline(always)]
+    fn queue_rq(queue_data: &QueueData, rq: ARef<mq::Request<Self>>, _is_last: bool) -> Result {
+        match queue_data.irq_mode {
+            IRQMode::None => mq::Request::end_ok(rq)
+                .map_err(|_e| kernel::error::code::EIO)
+                // We take no refcounts on the request, so we expect to be able to
+                // end the request. The request reference must be unique at this
+                // point, and so `end_ok` cannot fail.
+                .expect("Fatal error - expected to be able to end request"),
+            IRQMode::Soft => mq::Request::complete(rq),
+        }
+        Ok(())
+    }
+
+    fn commit_rqs(_queue_data: &QueueData) {}
+
+    fn complete(rq: ARef<mq::Request<Self>>) {
+        mq::Request::end_ok(rq)
+            .map_err(|_e| kernel::error::code::EIO)
+            // We take no refcounts on the request, so we expect to be able to
+            // end the request. The request reference must be unique at this
+            // point, and so `end_ok` cannot fail.
+            .expect("Fatal error - expected to be able to end request");
+    }
+}