Re: [PATCH 0/4] Drop wbinvd_on_all_cpus usage

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Hi, Tvrtko.

On 3/21/22 11:27, Tvrtko Ursulin wrote:

On 19/03/2022 19:42, Michael Cheng wrote:
To align with the discussion in [1][2], this patch series drops all usage of
wbvind_on_all_cpus within i915 by either replacing the call with certain
drm clflush helpers, or reverting to a previous logic.

AFAIU, complaint from [1] was that it is wrong to provide non x86 implementations under the wbinvd_on_all_cpus name. Instead an arch agnostic helper which achieves the same effect could be created. Does Arm have such concept?

I also understand Linus' email like we shouldn't leak incoherent IO to other architectures, meaning any remaining wbinvd()s should be X86 only.

Also, wbinvd_on_all_cpus() can become very costly, hence prefer the range apis when possible if they can be verified not to degrade performance.



Given that the series seems to be taking a different route, avoiding the need to call wbinvd_on_all_cpus rather than what [1] suggests (note drm_clflush_sg can still call it!?), concern is that the series has a bunch of reverts and each one needs to be analyzed.


Agreed.

/Thomas




For instance looking at just the last one, 64b95df91f44, who has looked at the locking consequences that commit describes:

"""
    Inside gtt_restore_mappings() we currently take the obj->resv->lock, but     in the future we need to avoid taking this fs-reclaim tainted lock as we
    need to extend the coverage of the vm->mutex. Take advantage of the
    single-threaded nature of the early resume phase, and do a single
    wbinvd() to flush all the GTT objects en masse.

"""

?

Then there are suspend and freeze reverts which presumably can regress the suspend times. Any data on those?

Adding Matt since he was the reviewer for that work so might remember something.

Regards,

Tvrtko


[1]. https://lists.freedesktop.org/archives/dri-devel/2021-November/330928.html
[2]. https://patchwork.freedesktop.org/patch/475752/?series=99991&rev=5

Michael Cheng (4):
   i915/gem: drop wbinvd_on_all_cpus usage
   Revert "drm/i915/gem: Almagamate clflushes on suspend"
   i915/gem: Revert i915_gem_freeze to previous logic
   drm/i915/gt: Revert ggtt_resume to previous logic

  drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c |  9 +---
  drivers/gpu/drm/i915/gem/i915_gem_pm.c     | 56 ++++++++++++++--------
  drivers/gpu/drm/i915/gt/intel_ggtt.c       | 17 +++----
  drivers/gpu/drm/i915/gt/intel_gtt.h        |  2 +-
  4 files changed, 46 insertions(+), 38 deletions(-)




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