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, 78 insertions, 0 deletions

diff --git a/Documentation/driver-api/wbrf.rst b/Documentation/driver-api/wbrf.rst
new file mode 100644
index 000000000..6b18833e2
--- /dev/null
+++ b/Documentation/driver-api/wbrf.rst
@@ -0,0 +1,78 @@
+.. SPDX-License-Identifier: GPL-2.0-or-later
+
+=================================
+WBRF - Wifi Band RFI Mitigations
+=================================
+
+Due to electrical and mechanical constraints in certain platform designs
+there may be likely interference of relatively high-powered harmonics of
+the GPU memory clocks with local radio module frequency bands used by
+certain Wifi bands.
+
+To mitigate possible RFI interference producers can advertise the
+frequencies in use and consumers can use this information to avoid using
+these frequencies for sensitive features.
+
+When a platform is known to have this issue with any contained devices,
+the platform designer will advertise the availability of this feature via
+ACPI devices with a device specific method (_DSM).
+* Producers with this _DSM will be able to advertise the frequencies in use.
+* Consumers with this _DSM will be able to register for notifications of
+frequencies in use.
+
+Some general terms
+==================
+
+Producer: such component who can produce high-powered radio frequency
+Consumer: such component who can adjust its in-use frequency in
+response to the radio frequencies of other components to mitigate the
+possible RFI.
+
+To make the mechanism function, those producers should notify active use
+of their particular frequencies so that other consumers can make relative
+internal adjustments as necessary to avoid this resonance.
+
+ACPI interface
+==============
+
+Although initially used by for wifi + dGPU use cases, the ACPI interface
+can be scaled to any type of device that a platform designer discovers
+can cause interference.
+
+The GUID used for the _DSM is 7B7656CF-DC3D-4C1C-83E9-66E721DE3070.
+
+3 functions are available in this _DSM:
+
+* 0: discover # of functions available
+* 1: record RF bands in use
+* 2: retrieve RF bands in use
+
+Driver programming interface
+============================
+
+.. kernel-doc:: drivers/platform/x86/amd/wbrf.c
+
+Sample Usage
+=============
+
+The expected flow for the producers:
+1. During probe, call `acpi_amd_wbrf_supported_producer` to check if WBRF
+can be enabled for the device.
+2. On using some frequency band, call `acpi_amd_wbrf_add_remove` with 'add'
+param to get other consumers properly notified.
+3. Or on stopping using some frequency band, call
+`acpi_amd_wbrf_add_remove` with 'remove' param to get other consumers notified.
+
+The expected flow for the consumers:
+1. During probe, call `acpi_amd_wbrf_supported_consumer` to check if WBRF
+can be enabled for the device.
+2. Call `amd_wbrf_register_notifier` to register for notification
+of frequency band change(add or remove) from other producers.
+3. Call the `amd_wbrf_retrieve_freq_band` initially to retrieve
+current active frequency bands considering some producers may broadcast
+such information before the consumer is up.
+4. On receiving a notification for frequency band change, run
+`amd_wbrf_retrieve_freq_band` again to retrieve the latest
+active frequency bands.
+5. During driver cleanup, call `amd_wbrf_unregister_notifier` to
+unregister the notifier.