[PATCH] drm/amdkfd: move flushing TLBs from map to unmap

[Felix Kuehling]

Similar to a recent fix by Philip Yang 76e08b37d0aa ("drm/amdgpu: flush
TLB if valid PDE turns into PTE"), there needs to be a conditional TLB
flush after map, if any PDEs were unmapped and turned into PTEs in the
process. This is currently returned by amdgpu_vm_bo_update_mapping in
the "table_freed" parameter. This needs to be also returned by
amdgpu_vm_bo_update and reported back to KFD, so KFD can do the TLB
flush after map, if needed.

kfd_flush_tlb probably needs a new parameter to determine the flush
type. The flush after map can be a "legacy" flush (type 0). The flush
after unmap must be a "heavy-weight" flush (type 2) to make sure we
don't evict cache lines into pages that we no longer own.

Finally, in the ticket I thought about possible optimizations using a
worker to minimize the impact of TLB flushes on unmap latency. That
could be a follow up commit.

Regards,
  Felix


Am 2021-05-25 um 1:53 p.m. schrieb Eric Huang:
> It it to optimize memory allocation latency.
>
> Signed-off-by: Eric Huang <jinhuieric.huang at amd.com>
>
> diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_chardev.c
> b/drivers/gpu/drm/amd/amdkfd/kfd_chardev.c
> index 960913a35ee4..ab73741edb97 100644
> --- a/drivers/gpu/drm/amd/amdkfd/kfd_chardev.c
> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_chardev.c
> @@ -1657,20 +1657,6 @@ static int kfd_ioctl_map_memory_to_gpu(struct
> file *filep,
>                 goto sync_memory_failed;
>         }
>
> -       /* Flush TLBs after waiting for the page table updates to
> complete */
> -       for (i = 0; i < args->n_devices; i++) {
> -               peer = kfd_device_by_id(devices_arr[i]);
> -               if (WARN_ON_ONCE(!peer))
> -                       continue;
> -               peer_pdd = kfd_get_process_device_data(peer, p);
> -               if (WARN_ON_ONCE(!peer_pdd))
> -                       continue;
> -               if (!amdgpu_read_lock(peer->ddev, true)) {
> -                       kfd_flush_tlb(peer_pdd);
> -                       amdgpu_read_unlock(peer->ddev);
> -               }
> -       }
> -
>         kfree(devices_arr);
>
>         trace_kfd_map_memory_to_gpu_end(p,
> @@ -1766,6 +1752,7 @@ static int
> kfd_ioctl_unmap_memory_from_gpu(struct file *filep,
>                         amdgpu_read_unlock(peer->ddev);
>                         goto unmap_memory_from_gpu_failed;
>                 }
> +               kfd_flush_tlb(peer_pdd);
>                 amdgpu_read_unlock(peer->ddev);
>                 args->n_success = i+1;
>         }
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