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, 135 insertions, 0 deletions

diff --git a/Documentation/input/devices/rotary-encoder.rst b/Documentation/input/devices/rotary-encoder.rst
new file mode 100644
index 000000000..5865748c1
--- /dev/null
+++ b/Documentation/input/devices/rotary-encoder.rst
@@ -0,0 +1,135 @@
+============================================================
+rotary-encoder - a generic driver for GPIO connected devices
+============================================================
+
+:Author: Daniel Mack <daniel@caiaq.de>, Feb 2009
+
+Function
+--------
+
+Rotary encoders are devices which are connected to the CPU or other
+peripherals with two wires. The outputs are phase-shifted by 90 degrees
+and by triggering on falling and rising edges, the turn direction can
+be determined.
+
+Some encoders have both outputs low in stable states, others also have
+a stable state with both outputs high (half-period mode) and some have
+a stable state in all steps (quarter-period mode).
+
+The phase diagram of these two outputs look like this::
+
+                  _____       _____       _____
+                 |     |     |     |     |     |
+  Channel A  ____|     |_____|     |_____|     |____
+
+                 :  :  :  :  :  :  :  :  :  :  :  :
+            __       _____       _____       _____
+              |     |     |     |     |     |     |
+  Channel B   |_____|     |_____|     |_____|     |__
+
+                 :  :  :  :  :  :  :  :  :  :  :  :
+  Event          a  b  c  d  a  b  c  d  a  b  c  d
+
+                |<-------->|
+	          one step
+
+                |<-->|
+	          one step (half-period mode)
+
+                |<>|
+	          one step (quarter-period mode)
+
+For more information, please see
+	https://en.wikipedia.org/wiki/Rotary_encoder
+
+
+Events / state machine
+----------------------
+
+In half-period mode, state a) and c) above are used to determine the
+rotational direction based on the last stable state. Events are reported in
+states b) and d) given that the new stable state is different from the last
+(i.e. the rotation was not reversed half-way).
+
+Otherwise, the following apply:
+
+a) Rising edge on channel A, channel B in low state
+	This state is used to recognize a clockwise turn
+
+b) Rising edge on channel B, channel A in high state
+	When entering this state, the encoder is put into 'armed' state,
+	meaning that there it has seen half the way of a one-step transition.
+
+c) Falling edge on channel A, channel B in high state
+	This state is used to recognize a counter-clockwise turn
+
+d) Falling edge on channel B, channel A in low state
+	Parking position. If the encoder enters this state, a full transition
+	should have happened, unless it flipped back on half the way. The
+	'armed' state tells us about that.
+
+Platform requirements
+---------------------
+
+As there is no hardware dependent call in this driver, the platform it is
+used with must support gpiolib. Another requirement is that IRQs must be
+able to fire on both edges.
+
+
+Board integration
+-----------------
+
+To use this driver in your system, register a platform_device with the
+name 'rotary-encoder' and associate the IRQs and some specific platform
+data with it. Because the driver uses generic device properties, this can
+be done either via device tree, ACPI, or using static board files, like in
+example below:
+
+::
+
+	/* board support file example */
+
+	#include <linux/input.h>
+	#include <linux/gpio/machine.h>
+	#include <linux/property.h>
+
+	#define GPIO_ROTARY_A 1
+	#define GPIO_ROTARY_B 2
+
+	static struct gpiod_lookup_table rotary_encoder_gpios = {
+		.dev_id = "rotary-encoder.0",
+		.table = {
+			GPIO_LOOKUP_IDX("gpio-0",
+					GPIO_ROTARY_A, NULL, 0, GPIO_ACTIVE_LOW),
+			GPIO_LOOKUP_IDX("gpio-0",
+					GPIO_ROTARY_B, NULL, 1, GPIO_ACTIVE_HIGH),
+			{ },
+		},
+	};
+
+	static const struct property_entry rotary_encoder_properties[] = {
+		PROPERTY_ENTRY_U32("rotary-encoder,steps-per-period", 24),
+		PROPERTY_ENTRY_U32("linux,axis",		      ABS_X),
+		PROPERTY_ENTRY_U32("rotary-encoder,relative_axis",    0),
+		{ },
+	};
+
+	static const struct software_node rotary_encoder_node = {
+		.properties = rotary_encoder_properties,
+	};
+
+	static struct platform_device rotary_encoder_device = {
+		.name		= "rotary-encoder",
+		.id		= 0,
+	};
+
+	...
+
+	gpiod_add_lookup_table(&rotary_encoder_gpios);
+	device_add_software_node(&rotary_encoder_device.dev, &rotary_encoder_node);
+	platform_device_register(&rotary_encoder_device);
+
+	...
+
+Please consult device tree binding documentation to see all properties
+supported by the driver.