Re: [Intel-gfx] [RFC PATCH] drm/ttm: Allow the driver to resolve a WW transaction rollback

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

On Fri, 2023-05-05 at 16:17 +0200, Thomas Hellström wrote:
> Allow drivers to resolve a WW transaction rollback. This allows for
> 1) Putting a lower-priority transaction to sleep allowing another to
> succeed instead both fighting using trylocks.
> 2) Letting the driver know whether a received -ENOMEM is the result
> of
> competition with another WW transaction, which can be resolved using
> rollback and retry or a real -ENOMEM which should be propagated back
> to user-space as a failure.
> 
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

Christian, Any objections?

/Thomas


> ---
>  drivers/gpu/drm/ttm/ttm_bo.c | 17 +++++++++++++++--
>  include/drm/ttm/ttm_bo.h     |  2 ++
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c
> b/drivers/gpu/drm/ttm/ttm_bo.c
> index bd5dae4d1624..c3ccbea2be3e 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -561,6 +561,10 @@ static int ttm_mem_evict_wait_busy(struct
> ttm_buffer_object *busy_bo,
>         if (!busy_bo || !ticket)
>                 return -EBUSY;
>  
> +       /* We want to resolve contention before trying to lock again.
> */
> +       if (ctx->propagate_edeadlk && ctx->contended_bo)
> +               return  -EDEADLK;
> +
>         if (ctx->interruptible)
>                 r = dma_resv_lock_interruptible(busy_bo->base.resv,
>                                                           ticket);
> @@ -575,7 +579,15 @@ static int ttm_mem_evict_wait_busy(struct
> ttm_buffer_object *busy_bo,
>         if (!r)
>                 dma_resv_unlock(busy_bo->base.resv);
>  
> -       return r == -EDEADLK ? -EBUSY : r;
> +       if (r == -EDEADLK) {
> +               if (ctx->propagate_edeadlk) {
> +                       ttm_bo_get(busy_bo);
> +                       ctx->contended_bo = busy_bo;
> +               }
> +               r = -EBUSY;
> +       }
> +
> +       return r;
>  }
>  
>  int ttm_mem_evict_first(struct ttm_device *bdev,
> @@ -816,7 +828,7 @@ int ttm_bo_mem_space(struct ttm_buffer_object
> *bo,
>                         goto error;
>         }
>  
> -       ret = -ENOMEM;
> +       ret = (ctx->propagate_edeadlk && ctx->contended_bo) ? -
> EDEADLK : -ENOMEM;
>         if (!type_found) {
>                 pr_err(TTM_PFX "No compatible memory type found\n");
>                 ret = -EINVAL;
> @@ -913,6 +925,7 @@ int ttm_bo_validate(struct ttm_buffer_object *bo,
>                 if (ret)
>                         return ret;
>         }
> +
>         return 0;
>  }
>  EXPORT_SYMBOL(ttm_bo_validate);
> diff --git a/include/drm/ttm/ttm_bo.h b/include/drm/ttm/ttm_bo.h
> index 8b113c384236..d8e35a794ce5 100644
> --- a/include/drm/ttm/ttm_bo.h
> +++ b/include/drm/ttm/ttm_bo.h
> @@ -181,8 +181,10 @@ struct ttm_operation_ctx {
>         bool gfp_retry_mayfail;
>         bool allow_res_evict;
>         bool force_alloc;
> +       bool propagate_edeadlk;
>         struct dma_resv *resv;
>         uint64_t bytes_moved;
> +       struct ttm_buffer_object *contended_bo;
>  };
>  
>  /**