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, 56 insertions, 0 deletions

diff --git a/Documentation/gpu/panthor.rst b/Documentation/gpu/panthor.rst
new file mode 100644
index 000000000..7a8417412
--- /dev/null
+++ b/Documentation/gpu/panthor.rst
@@ -0,0 +1,56 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+=========================
+ drm/Panthor CSF driver
+=========================
+
+.. _panthor-usage-stats:
+
+Panthor DRM client usage stats implementation
+==============================================
+
+The drm/Panthor driver implements the DRM client usage stats specification as
+documented in :ref:`drm-client-usage-stats`.
+
+Example of the output showing the implemented key value pairs and entirety of
+the currently possible format options:
+
+::
+     pos:    0
+     flags:  02400002
+     mnt_id: 29
+     ino:    491
+     drm-driver:     panthor
+     drm-client-id:  10
+     drm-engine-panthor:     111110952750 ns
+     drm-cycles-panthor:     94439687187
+     drm-maxfreq-panthor:    1000000000 Hz
+     drm-curfreq-panthor:    1000000000 Hz
+     panthor-resident-memory:        10396 KiB
+     panthor-active-memory:  10396 KiB
+     drm-total-memory:       16480 KiB
+     drm-shared-memory:      0
+     drm-active-memory:      16200 KiB
+     drm-resident-memory:    16480 KiB
+     drm-purgeable-memory:   0
+
+Possible `drm-engine-` key names are: `panthor`.
+`drm-curfreq-` values convey the current operating frequency for that engine.
+
+Users must bear in mind that engine and cycle sampling are disabled by default,
+because of power saving concerns. `fdinfo` users and benchmark applications which
+query the fdinfo file must make sure to toggle the job profiling status of the
+driver by writing into the appropriate sysfs node::
+
+    echo <N> > /sys/bus/platform/drivers/panthor/[a-f0-9]*.gpu/profiling
+
+Where `N` is a bit mask where cycle and timestamp sampling are respectively
+enabled by the first and second bits.
+
+Possible `panthor-*-memory` keys are: `active` and `resident`.
+These values convey the sizes of the internal driver-owned shmem BO's that
+aren't exposed to user-space through a DRM handle, like queue ring buffers,
+sync object arrays and heap chunks. Because they are all allocated and pinned
+at creation time, only `panthor-resident-memory` is necessary to tell us their
+size. `panthor-active-memory` shows the size of kernel BO's associated with
+VM's and groups currently being scheduled for execution by the GPU.