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, 105 insertions, 0 deletions

diff --git a/Documentation/gpu/amdgpu/debugging.rst b/Documentation/gpu/amdgpu/debugging.rst
new file mode 100644
index 000000000..ac914d524
--- /dev/null
+++ b/Documentation/gpu/amdgpu/debugging.rst
@@ -0,0 +1,105 @@
+===============
+ GPU Debugging
+===============
+
+General Debugging Options
+=========================
+
+The DebugFS section provides documentation on a number files to aid in debugging
+issues on the GPU.
+
+
+GPUVM Debugging
+===============
+
+To aid in debugging GPU virtual memory related problems, the driver supports a
+number of options module parameters:
+
+`vm_fault_stop` - If non-0, halt the GPU memory controller on a GPU page fault.
+
+`vm_update_mode` - If non-0, use the CPU to update GPU page tables rather than
+the GPU.
+
+
+Decoding a GPUVM Page Fault
+===========================
+
+If you see a GPU page fault in the kernel log, you can decode it to figure
+out what is going wrong in your application.  A page fault in your kernel
+log may look something like this:
+
+::
+
+ [gfxhub0] no-retry page fault (src_id:0 ring:24 vmid:3 pasid:32777, for process glxinfo pid 2424 thread glxinfo:cs0 pid 2425)
+   in page starting at address 0x0000800102800000 from IH client 0x1b (UTCL2)
+ VM_L2_PROTECTION_FAULT_STATUS:0x00301030
+ 	Faulty UTCL2 client ID: TCP (0x8)
+ 	MORE_FAULTS: 0x0
+ 	WALKER_ERROR: 0x0
+ 	PERMISSION_FAULTS: 0x3
+ 	MAPPING_ERROR: 0x0
+ 	RW: 0x0
+
+First you have the memory hub, gfxhub and mmhub.  gfxhub is the memory
+hub used for graphics, compute, and sdma on some chips.  mmhub is the
+memory hub used for multi-media and sdma on some chips.
+
+Next you have the vmid and pasid.  If the vmid is 0, this fault was likely
+caused by the kernel driver or firmware.  If the vmid is non-0, it is generally
+a fault in a user application.  The pasid is used to link a vmid to a system
+process id.  If the process is active when the fault happens, the process
+information will be printed.
+
+The GPU virtual address that caused the fault comes next.
+
+The client ID indicates the GPU block that caused the fault.
+Some common client IDs:
+
+- CB/DB: The color/depth backend of the graphics pipe
+- CPF: Command Processor Frontend
+- CPC: Command Processor Compute
+- CPG: Command Processor Graphics
+- TCP/SQC/SQG: Shaders
+- SDMA: SDMA engines
+- VCN: Video encode/decode engines
+- JPEG: JPEG engines
+
+PERMISSION_FAULTS describe what faults were encountered:
+
+- bit 0: the PTE was not valid
+- bit 1: the PTE read bit was not set
+- bit 2: the PTE write bit was not set
+- bit 3: the PTE execute bit was not set
+
+Finally, RW, indicates whether the access was a read (0) or a write (1).
+
+In the example above, a shader (cliend id = TCP) generated a read (RW = 0x0) to
+an invalid page (PERMISSION_FAULTS = 0x3) at GPU virtual address
+0x0000800102800000.  The user can then inspect their shader code and resource
+descriptor state to determine what caused the GPU page fault.
+
+UMR
+===
+
+`umr <https://gitlab.freedesktop.org/tomstdenis/umr>`_ is a general purpose
+GPU debugging and diagnostics tool.  Please see the umr
+`documentation <https://umr.readthedocs.io/en/main/>`_ for more information
+about its capabilities.
+
+Debugging backlight brightness
+==============================
+Default backlight brightness is intended to be set via the policy advertised
+by the firmware.  Firmware will often provide different defaults for AC or DC.
+Furthermore, some userspace software will save backlight brightness during
+the previous boot and attempt to restore it.
+
+Some firmware also has support for a feature called "Custom Backlight Curves"
+where an input value for brightness is mapped along a linearly interpolated
+curve of brightness values that better match display characteristics.
+
+In the event of problems happening with backlight, there is a trace event
+that can be enabled at bootup to log every brightness change request.
+This can help isolate where the problem is. To enable the trace event add
+the following to the kernel command line:
+
+  tp_printk trace_event=amdgpu_dm:amdgpu_dm_brightness:mod:amdgpu trace_buf_size=1M