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, 62 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-class-led-trigger-tty b/Documentation/ABI/testing/sysfs-class-led-trigger-tty
new file mode 100644
index 000000000..308fbc362
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-led-trigger-tty
@@ -0,0 +1,62 @@
+What:		/sys/class/leds/<tty_led>/ttyname
+Date:		Dec 2020
+KernelVersion:	5.10
+Contact:	linux-leds@vger.kernel.org
+Description:
+		Specifies the tty device name of the triggering tty
+
+What:		/sys/class/leds/<tty_led>/rx
+Date:		February 2024
+KernelVersion:	6.8
+Description:
+		Signal reception (rx) of data on the named tty device.
+		If set to 0, the LED will not blink on reception.
+		If set to 1 (default), the LED will blink on reception.
+
+What:		/sys/class/leds/<tty_led>/tx
+Date:		February 2024
+KernelVersion:	6.8
+Description:
+		Signal transmission (tx) of data on the named tty device.
+		If set to 0, the LED will not blink on transmission.
+		If set to 1 (default), the LED will blink on transmission.
+
+What:		/sys/class/leds/<tty_led>/cts
+Date:		February 2024
+KernelVersion:	6.8
+Description:
+		CTS = Clear To Send
+		DCE is ready to accept data from the DTE.
+		If the line state is detected, the LED is switched on.
+		If set to 0 (default), the LED will not evaluate CTS.
+		If set to 1, the LED will evaluate CTS.
+
+What:		/sys/class/leds/<tty_led>/dsr
+Date:		February 2024
+KernelVersion:	6.8
+Description:
+		DSR = Data Set Ready
+		DCE is ready to receive and send data.
+		If the line state is detected, the LED is switched on.
+		If set to 0 (default), the LED will not evaluate DSR.
+		If set to 1, the LED will evaluate DSR.
+
+What:		/sys/class/leds/<tty_led>/dcd
+Date:		February 2024
+KernelVersion:	6.8
+Description:
+		DCD = Data Carrier Detect
+		DTE is receiving a carrier from the DCE.
+		If the line state is detected, the LED is switched on.
+		If set to 0 (default), the LED will not evaluate CAR (DCD).
+		If set to 1, the LED will evaluate CAR (DCD).
+
+What:		/sys/class/leds/<tty_led>/rng
+Date:		February 2024
+KernelVersion:	6.8
+Description:
+		RNG = Ring Indicator
+		DCE has detected an incoming ring signal on the telephone
+		line. If the line state is detected, the LED is switched on.
+		If set to 0 (default), the LED will not evaluate RNG.
+		If set to 1, the LED will evaluate RNG.