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, 99 insertions, 0 deletions

diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence
new file mode 100644
index 000000000..6fbbebec6
--- /dev/null
+++ b/lib/Kconfig.kfence
@@ -0,0 +1,99 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+config HAVE_ARCH_KFENCE
+	bool
+
+menuconfig KFENCE
+	bool "KFENCE: low-overhead sampling-based memory safety error detector"
+	depends on HAVE_ARCH_KFENCE
+	select STACKTRACE
+	select IRQ_WORK
+	help
+	  KFENCE is a low-overhead sampling-based detector of heap out-of-bounds
+	  access, use-after-free, and invalid-free errors. KFENCE is designed
+	  to have negligible cost to permit enabling it in production
+	  environments.
+
+	  See <file:Documentation/dev-tools/kfence.rst> for more details.
+
+	  Note that, KFENCE is not a substitute for explicit testing with tools
+	  such as KASAN. KFENCE can detect a subset of bugs that KASAN can
+	  detect, albeit at very different performance profiles. If you can
+	  afford to use KASAN, continue using KASAN, for example in test
+	  environments. If your kernel targets production use, and cannot
+	  enable KASAN due to its cost, consider using KFENCE.
+
+if KFENCE
+
+config KFENCE_SAMPLE_INTERVAL
+	int "Default sample interval in milliseconds"
+	default 100
+	help
+	  The KFENCE sample interval determines the frequency with which heap
+	  allocations will be guarded by KFENCE. May be overridden via boot
+	  parameter "kfence.sample_interval".
+
+	  Set this to 0 to disable KFENCE by default, in which case only
+	  setting "kfence.sample_interval" to a non-zero value enables KFENCE.
+
+config KFENCE_NUM_OBJECTS
+	int "Number of guarded objects available"
+	range 1 65535
+	default 255
+	help
+	  The number of guarded objects available. For each KFENCE object, 2
+	  pages are required; with one containing the object and two adjacent
+	  ones used as guard pages.
+
+config KFENCE_DEFERRABLE
+	bool "Use a deferrable timer to trigger allocations"
+	help
+	  Use a deferrable timer to trigger allocations. This avoids forcing
+	  CPU wake-ups if the system is idle, at the risk of a less predictable
+	  sample interval.
+
+	  Warning: The KUnit test suite fails with this option enabled - due to
+	  the unpredictability of the sample interval!
+
+	  Say N if you are unsure.
+
+config KFENCE_STATIC_KEYS
+	bool "Use static keys to set up allocations" if EXPERT
+	depends on JUMP_LABEL
+	help
+	  Use static keys (static branches) to set up KFENCE allocations. This
+	  option is only recommended when using very large sample intervals, or
+	  performance has carefully been evaluated with this option.
+
+	  Using static keys comes with trade-offs that need to be carefully
+	  evaluated given target workloads and system architectures. Notably,
+	  enabling and disabling static keys invoke IPI broadcasts, the latency
+	  and impact of which is much harder to predict than a dynamic branch.
+
+	  Say N if you are unsure.
+
+config KFENCE_STRESS_TEST_FAULTS
+	int "Stress testing of fault handling and error reporting" if EXPERT
+	default 0
+	help
+	  The inverse probability with which to randomly protect KFENCE object
+	  pages, resulting in spurious use-after-frees. The main purpose of
+	  this option is to stress test KFENCE with concurrent error reports
+	  and allocations/frees. A value of 0 disables stress testing logic.
+
+	  Only for KFENCE testing; set to 0 if you are not a KFENCE developer.
+
+config KFENCE_KUNIT_TEST
+	tristate "KFENCE integration test suite" if !KUNIT_ALL_TESTS
+	default KUNIT_ALL_TESTS
+	depends on TRACEPOINTS && KUNIT
+	help
+	  Test suite for KFENCE, testing various error detection scenarios with
+	  various allocation types, and checking that reports are correctly
+	  output to console.
+
+	  Say Y here if you want the test to be built into the kernel and run
+	  during boot; say M if you want the test to build as a module; say N
+	  if you are unsure.
+
+endif # KFENCE