Re: [PATCH v4 00/12] Fine grained fault locking, threaded prefetch, storm cache

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

Hi, Matt.

On Wed, 2026-02-25 at 20:28 -0800, Matthew Brost wrote:
> Fine-grained fault locking provides immediate benefits: it allows
> page
> faults from the same VM to be processed in parallel (unless they
> target
> the same range) and enables a sane multi-threaded prefetch
> implementation. UMD prefetch benchmarks see 10% to 50% improvement in
> prefetch performance on BMG depending on PCIe bus speed.
> 
> Once parallel fault processing is available, the pagefault queue can
> be
> unified into a single queue with multiple workers pulling faults to
> process. A single queue then allows a sensible pagefault cache to be
> implemented, so that multiple faults targeting the same region can be
> batched together and acknowledged in, ideally, a single pass. This
> saves
> CPU cycles during pagefault handling and improves overall throughput
> of
> the fault handler.
> 
> Significant improvements in UMD pagefault benchmarks can be seen when
> utilizing this caching.
> 
> v3:
>  - Fix kunit build (CI)
> v4:
>  - Actually fix kunit build (CI)
> 
> Matt
> 
> Matthew Brost (12):
>   drm/xe: Fine grained page fault locking
>   drm/xe: Allow prefetch-only VM bind IOCTLs to use VM read lock
>   drm/xe: Thread prefetch of SVM ranges
>   drm/xe: Use a single page-fault queue with multiple workers
>   drm/xe: Add num_pf_work modparam
>   drm/xe: Engine class and instance into a u8
>   drm/xe: Track pagefault worker runtime
>   drm/xe: Chain page faults via queue-resident cache to avoid fault
>     storms
>   drm/xe: Add pagefault chaining stats
>   drm/xe: Add debugfs pagefault_info
>   drm/xe: batch CT pagefault acks with periodic flush
>   drm/xe: Track parallel page fault activity in GT stats
> 
>  drivers/gpu/drm/drm_gpusvm.c            |   2 +-
>  drivers/gpu/drm/xe/xe_debugfs.c         |  11 +
>  drivers/gpu/drm/xe/xe_defaults.h        |   1 +
>  drivers/gpu/drm/xe/xe_device.c          |  17 +-
>  drivers/gpu/drm/xe/xe_device_types.h    |  17 +-
>  drivers/gpu/drm/xe/xe_gt_stats.c        |   7 +
>  drivers/gpu/drm/xe/xe_gt_stats_types.h  |   7 +
>  drivers/gpu/drm/xe/xe_guc_ct.c          |  94 +++-
>  drivers/gpu/drm/xe/xe_guc_ct.h          |  35 +-
>  drivers/gpu/drm/xe/xe_guc_pagefault.c   |  35 +-
>  drivers/gpu/drm/xe/xe_guc_types.h       |   6 +
>  drivers/gpu/drm/xe/xe_module.c          |   4 +
>  drivers/gpu/drm/xe/xe_module.h          |   1 +
>  drivers/gpu/drm/xe/xe_pagefault.c       | 675 ++++++++++++++++++++--
> --
>  drivers/gpu/drm/xe/xe_pagefault.h       |  74 +++
>  drivers/gpu/drm/xe/xe_pagefault_types.h | 109 +++-
>  drivers/gpu/drm/xe/xe_svm.c             | 129 +++--
>  drivers/gpu/drm/xe/xe_svm.h             |  59 ++-
>  drivers/gpu/drm/xe/xe_userptr.c         |  20 +-
>  drivers/gpu/drm/xe/xe_vm.c              | 215 ++++++--
>  drivers/gpu/drm/xe/xe_vm_types.h        |  37 +-
>  21 files changed, 1309 insertions(+), 246 deletions(-)

Before I get to reviewing this, some suggestions from Claude:

  Confirmed regressions (3 commits with issues):

  c664c1b91090 — Fine grained page fault locking

   - Reference leak in vm_bind_ioctl_ops_create (xe_vm.c). 
  xe_svm_range_find_or_insert() was changed to take a reference, but
two paths
   don't put it: (1) when xe_svm_range_validate() returns true → goto 
  check_next_range, and (2) when xa_alloc() fails → goto
unwind_prefetch_ops. 
  The validate path is on every prefetch of an already-populated range,
so 
  refcounts grow unbounded.

  80012f80c75f — Chain page faults

   - Commit message typos only: "samr ASID" → "same ASID", "IRQ pathd"
→ "IRQ 
  paths". No code issues.

  569104fb76ed — batch CT pagefault acks with periodic flush

   - Off-by-one in flush period: guc_ack_fault_begin initialises 
  pagefault_ack_counter to PERIOD - 2 = 14, but the comment says first
flush 
  should be at ack #2. With counter=14 the first flush fires at ack #3 
  (counter hits 16, 16&15==0). Fix: = XE_GUC_PAGEFAULT_FLUSH_PERIOD -
1.
   - Commit message typo: "Assistent-by" → "Assisted-by".

/Thomas