On Thu, 2026-02-05 at 22:20 +1100, Balbir Singh wrote: > On 2/5/26 22:10, Thomas Hellström wrote: > > If hmm_range_fault() fails a folio_trylock() in do_swap_page, > > trying to acquire the lock of a device-private folio for migration, > > to ram, the function will spin until it succeeds grabbing the lock. > > > > However, if the process holding the lock is depending on a work > > item to be completed, which is scheduled on the same CPU as the > > spinning hmm_range_fault(), that work item might be starved and > > we end up in a livelock / starvation situation which is never > > resolved. > > > > This can happen, for example if the process holding the > > device-private folio lock is stuck in > > migrate_device_unmap()->lru_add_drain_all() > > The lru_add_drain_all() function requires a short work-item > > to be run on all online cpus to complete. > > > > A prerequisite for this to happen is: > > a) Both zone device and system memory folios are considered in > > migrate_device_unmap(), so that there is a reason to call > > lru_add_drain_all() for a system memory folio while a > > folio lock is held on a zone device folio. > > b) The zone device folio has an initial mapcount > 1 which causes > > at least one migration PTE entry insertion to be deferred to > > try_to_migrate(), which can happen after the call to > > lru_add_drain_all(). > > c) No or voluntary only preemption. > > > > This all seems pretty unlikely to happen, but indeed is hit by > > the "xe_exec_system_allocator" igt test. > > > > Do you have a stack trace from the test? I am trying to visualize the > livelock/starvation, but I can't from the description.
The spinning thread: (The backtrace varies slightly from time to time:) [ 805.201476] watchdog: BUG: soft lockup - CPU#139 stuck for 52s! [kworker/u900:1:9985] [ 805.201477] Modules linked in: xt_conntrack nft_chain_nat xt_MASQUERADE nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 bridge stp llc xfrm_user xfrm_algo xt_addrtype nft_compat x_tables nf_tables mei_gsc_proxy pmt_crashlog mtd_intel_dg mei_gsc overlay qrtr snd_hda_codec_intelhdmi snd_hda_codec_hdmi intel_rapl_msr intel_rapl_common cfg80211 intel_uncore_frequency intel_uncore_frequency_common intel_ifs i10nm_edac sunrpc binfmt_misc skx_edac_common nfit xe x86_pkg_temp_thermal intel_powerclamp coretemp nls_iso8859_1 kvm_intel kvm drm_ttm_helper drm_suballoc_helper gpu_sched snd_hda_intel cmdlinepart drm_gpuvm snd_intel_dspcfg drm_exec spi_nor drm_gpusvm_helper snd_hda_codec drm_buddy pmt_telemetry dax_hmem snd_hwdep pmt_discovery mtd video irqbypass cxl_acpi qat_4xxx iaa_crypto snd_hda_core pmt_class ttm rapl ses cxl_port snd_pcm intel_cstate enclosure cxl_core intel_qat isst_if_mmio isst_if_mbox_pci drm_display_helper snd_timer snd cec idxd crc8 einj ast mei_me spi_intel_pci rc_core soundcore isst_if_common [ 805.201496] ipmi_ssif authenc i2c_i801 intel_vsec idxd_bus spi_intel i2c_algo_bit mei i2c_ismt i2c_smbus wmi joydev input_leds ipmi_si acpi_power_meter acpi_ipmi ipmi_devintf ipmi_msghandler acpi_pad mac_hid pfr_telemetry pfr_update sch_fq_codel msr efi_pstore dm_multipath nfnetlink dmi_sysfs autofs4 btrfs blake2b libblake2b raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq raid1 raid0 linear rndis_host cdc_ether usbnet mii nvme hid_generic mpt3sas i40e nvme_core usbhid ahci ghash_clmulni_intel raid_class nvme_keyring scsi_transport_sas hid libahci nvme_auth libie hkdf libie_adminq pinctrl_emmitsburg aesni_intel [ 805.201510] CPU: 139 UID: 0 PID: 9985 Comm: kworker/u900:1 Tainted: G S W L 6.19.0-rc7+ #18 PREEMPT(voluntary) [ 805.201512] Tainted: [S]=CPU_OUT_OF_SPEC, [W]=WARN, [L]=SOFTLOCKUP [ 805.201512] Hardware name: Supermicro SYS-421GE-TNRT/X13DEG-OA, BIOS 2.5a 02/21/2025 [ 805.201513] Workqueue: xe_page_fault_work_queue xe_pagefault_queue_work [xe] [ 805.201599] RIP: 0010:_raw_spin_unlock+0x16/0x40 [ 805.201602] Code: cc 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 44 00 00 55 48 89 e5 c6 07 00 0f 1f 00 65 ff 0d fa a6 40 01 <74> 10 5d 31 c0 31 d2 31 c9 31 f6 31 ff c3 cc cc cc cc 0f 1f 44 00 [ 805.201603] RSP: 0018:ffffd2a663a4f678 EFLAGS: 00000247 [ 805.201603] RAX: fffff85c67e35080 RBX: ffffd2a663a4f7b8 RCX: 0000000000000000 [ 805.201604] RDX: ffff8b88fdd31a00 RSI: 0000000000000000 RDI: fffff75c86ff5928 [ 805.201605] RBP: ffffd2a663a4f678 R08: 0000000000000000 R09: 0000000000000000 [ 805.201605] R10: 0000000000000000 R11: 0000000000000000 R12: 0000631d10d42000 [ 805.201606] R13: ffffd2a663a4f7b8 R14: 00000001a4ca4067 R15: 74000003ff9f8d42 [ 805.201606] FS: 0000000000000000(0000) GS:ffff8bc76202b000(0000) knlGS:0000000000000000 [ 805.201607] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 805.201608] CR2: 0000631d10c00088 CR3: 0000003de3040004 CR4: 0000000000f72ef0 [ 805.201609] PKRU: 55555554 [ 805.201609] Call Trace: [ 805.201610] <TASK> [ 805.201610] do_swap_page+0x17c6/0x1b70 [ 805.201612] ? sysvec_apic_timer_interrupt+0x57/0xc0 [ 805.201614] ? asm_sysvec_apic_timer_interrupt+0x1b/0x20 [ 805.201615] ? __pfx_default_wake_function+0x10/0x10 [ 805.201617] ? ___pte_offset_map+0x1c/0x130 [ 805.201619] __handle_mm_fault+0xa75/0x1020 [ 805.201621] handle_mm_fault+0xeb/0x2f0 [ 805.201622] ? handle_mm_fault+0x11a/0x2f0 [ 805.201623] hmm_vma_fault.isra.0+0x5b/0xb0 [ 805.201625] hmm_vma_walk_pmd+0x5c7/0xc40 [ 805.201627] ? sysvec_apic_timer_interrupt+0x57/0xc0 [ 805.201629] walk_pgd_range+0x5ba/0xbf0 [ 805.201631] __walk_page_range+0x8e/0x220 [ 805.201633] walk_page_range_mm_unsafe+0x149/0x210 [ 805.201635] walk_page_range+0x2a/0x40 [ 805.201636] hmm_range_fault+0x5c/0xb0 [ 805.201638] drm_gpusvm_range_evict+0x11a/0x1d0 [drm_gpusvm_helper] [ 805.201641] __xe_svm_handle_pagefault+0x5fa/0xf00 [xe] [ 805.201736] ? select_task_rq_fair+0x9bc/0x2970 [ 805.201738] xe_svm_handle_pagefault+0x3d/0xb0 [xe] [ 805.201827] xe_pagefault_queue_work+0x233/0x370 [xe] [ 805.201905] process_one_work+0x18d/0x370 [ 805.201907] worker_thread+0x31a/0x460 [ 805.201908] ? __pfx_worker_thread+0x10/0x10 [ 805.201909] kthread+0x10b/0x220 [ 805.201910] ? __pfx_kthread+0x10/0x10 [ 805.201912] ret_from_fork+0x289/0x2c0 [ 805.201913] ? __pfx_kthread+0x10/0x10 [ 805.201915] ret_from_fork_asm+0x1a/0x30 [ 805.201917] </TASK> The thread holding the page-lock: [ 1629.938195] Workqueue: xe_page_fault_work_queue xe_pagefault_queue_work [xe] [ 1629.938340] Call Trace: [ 1629.938341] <TASK> [ 1629.938342] __schedule+0x47f/0x1890 [ 1629.938346] ? psi_group_change+0x1bd/0x4d0 [ 1629.938350] ? __pick_eevdf+0x70/0x180 [ 1629.938353] schedule+0x27/0xf0 [ 1629.938357] schedule_timeout+0xcf/0x110 [ 1629.938361] __wait_for_common+0x98/0x180 [ 1629.938364] ? __pfx_schedule_timeout+0x10/0x10 [ 1629.938368] wait_for_completion+0x24/0x40 [ 1629.938370] __flush_work+0x2b6/0x400 [ 1629.938373] ? kick_pool+0x77/0x1b0 [ 1629.938377] ? __pfx_wq_barrier_func+0x10/0x10 [ 1629.938382] flush_work+0x1c/0x30 [ 1629.938384] __lru_add_drain_all+0x19f/0x2a0 [ 1629.938390] lru_add_drain_all+0x10/0x20 [ 1629.938392] migrate_device_unmap+0x433/0x480 [ 1629.938398] migrate_vma_setup+0x245/0x300 [ 1629.938403] drm_pagemap_migrate_to_devmem+0x2a8/0xc00 [drm_gpusvm_helper] [ 1629.938410] ? krealloc_node_align_noprof+0x12f/0x3a0 [ 1629.938413] ? __xe_bo_create_locked+0x376/0x840 [xe] [ 1629.938529] xe_drm_pagemap_populate_mm+0x25f/0x3a0 [xe] [ 1629.938721] drm_pagemap_populate_mm+0x74/0xe0 [drm_gpusvm_helper] [ 1629.938731] xe_svm_alloc_vram+0xad/0x270 [xe] [ 1629.938933] ? xe_tile_local_pagemap+0x41/0x170 [xe] [ 1629.939095] ? ktime_get+0x41/0x100 [ 1629.939098] __xe_svm_handle_pagefault+0xa90/0xf00 [xe] [ 1629.939279] xe_svm_handle_pagefault+0x3d/0xb0 [xe] [ 1629.939460] xe_pagefault_queue_work+0x233/0x370 [xe] [ 1629.939620] process_one_work+0x18d/0x370 [ 1629.939623] worker_thread+0x31a/0x460 [ 1629.939626] ? __pfx_worker_thread+0x10/0x10 [ 1629.939629] kthread+0x10b/0x220 [ 1629.939632] ? __pfx_kthread+0x10/0x10 [ 1629.939636] ret_from_fork+0x289/0x2c0 [ 1629.939639] ? __pfx_kthread+0x10/0x10 [ 1629.939642] ret_from_fork_asm+0x1a/0x30 [ 1629.939648] </TASK> The worker that this thread waits on in flush_work() is, most likely, the one starved on cpu-time on cpu #139. Thanks, Thomas
