From 0d6526f82c3cdefcca47f73f5fc08dc6f335eac6 Mon Sep 17 00:00:00 2001 From: Tim Chen Date: Mon, 21 Sep 2026 17:37:22 -0700 Subject: sched/cache: Keep nr_pref_llc_running in the runnable domain, to fix LLC mis-scheduling bug 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 Suggested-by: Chen Yu Signed-off-by: Tim Chen Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Reviewed-by: Kayra Cizmeci Cc: # v7.2.x Link: https://patch.msgid.link/06af61afedac32e6477f57feb4d658f6c411c3af.1790035273.git.tim.c.chen@linux.intel.com --- Documentation/gpu/amdgpu/debugging.rst | 105 +++++++++++++++++++++++++++++++++ 1 file changed, 105 insertions(+) create mode 100644 Documentation/gpu/amdgpu/debugging.rst (limited to 'Documentation/gpu/amdgpu/debugging.rst') 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 `_ is a general purpose +GPU debugging and diagnostics tool. Please see the umr +`documentation `_ 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 -- cgit v1.3.1