[RFC PATCH 4/5] drm/ttm: Document and optimize ttm_bo_pipeline_gutting()
Thomas Hellström
thomas.hellstrom at linux.intel.com
Fri May 21 08:43:17 UTC 2021
On 5/21/21 10:21 AM, Christian König wrote:
> Am 20.05.21 um 17:09 schrieb Thomas Hellström:
>> If the bo is idle when calling ttm_bo_pipeline_gutting(), we
>> unnecessarily
>> create a ghost object and push it out to delayed destroy.
>> Fix this by adding a path for idle, and document the function.
>>
>> Also avoid having the bo end up in a bad state vulnerable to user-space
>> triggered kernel BUGs if the call to ttm_tt_create() fails.
>>
>> Finally reuse ttm_bo_pipeline_gutting() in ttm_bo_evict().
>>
>> Cc: Christian König <christian.koenig at amd.com>
>> Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
>> ---
>> drivers/gpu/drm/ttm/ttm_bo.c | 20 +++++-----
>> drivers/gpu/drm/ttm/ttm_bo_util.c | 63 ++++++++++++++++++++++++-------
>> drivers/gpu/drm/ttm/ttm_tt.c | 5 +++
>> include/drm/ttm/ttm_tt.h | 10 +++++
>> 4 files changed, 75 insertions(+), 23 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
>> index ca1b098b6a56..a8fa3375b8aa 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>> @@ -501,10 +501,15 @@ static int ttm_bo_evict(struct
>> ttm_buffer_object *bo,
>> bdev->funcs->evict_flags(bo, &placement);
>> if (!placement.num_placement && !placement.num_busy_placement) {
>> - ttm_bo_wait(bo, false, false);
>> + ret = ttm_bo_wait(bo, true, false);
>> + if (ret)
>> + return ret;
>> - ttm_bo_cleanup_memtype_use(bo);
>> - return ttm_tt_create(bo, false);
>> + /*
>> + * Since we've already synced, this frees backing store
>> + * immediately.
>> + */
>> + return ttm_bo_pipeline_gutting(bo);
>
> Yeah, we tried to avoid pipeline_gutting here because of eviction. But
> I think when you wait before that should work.
>
>> }
>> ret = ttm_bo_mem_space(bo, &placement, &evict_mem, ctx);
>> @@ -974,13 +979,8 @@ int ttm_bo_validate(struct ttm_buffer_object *bo,
>> /*
>> * Remove the backing store if no placement is given.
>> */
>> - if (!placement->num_placement && !placement->num_busy_placement) {
>> - ret = ttm_bo_pipeline_gutting(bo);
>> - if (ret)
>> - return ret;
>> -
>> - return ttm_tt_create(bo, false);
>> - }
>> + if (!placement->num_placement && !placement->num_busy_placement)
>> + return ttm_bo_pipeline_gutting(bo);
>> /*
>> * Check whether we need to move buffer.
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c
>> b/drivers/gpu/drm/ttm/ttm_bo_util.c
>> index 919ee03f7eb3..1860e2e7563f 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo_util.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
>> @@ -479,7 +479,8 @@ static void ttm_transfered_destroy(struct
>> ttm_buffer_object *bo)
>> */
>> static int ttm_buffer_object_transfer(struct ttm_buffer_object *bo,
>> - struct ttm_buffer_object **new_obj)
>> + struct ttm_buffer_object **new_obj,
>> + bool realloc_tt)
>> {
>> struct ttm_transfer_obj *fbo;
>> int ret;
>> @@ -493,6 +494,17 @@ static int ttm_buffer_object_transfer(struct
>> ttm_buffer_object *bo,
>> ttm_bo_get(bo);
>> fbo->bo = bo;
>> + if (realloc_tt) {
>> + bo->ttm = NULL;
>> + ret = ttm_tt_create(bo, true);
>> + if (ret) {
>> + bo->ttm = fbo->base.ttm;
>> + kfree(fbo);
>> + ttm_bo_put(bo);
>> + return ret;
>> + }
>> + }
>> +
>
> Can't we keep that logic in the caller? I think that would be cleaner.
Indeed, let me see if we can do that without breaking anything.
>
>> /**
>> * Fix up members that we shouldn't copy directly:
>> * TODO: Explicit member copy would probably be better here.
>> @@ -763,7 +775,7 @@ static int ttm_bo_move_to_ghost(struct
>> ttm_buffer_object *bo,
>> dma_fence_put(bo->moving);
>> bo->moving = dma_fence_get(fence);
>> - ret = ttm_buffer_object_transfer(bo, &ghost_obj);
>> + ret = ttm_buffer_object_transfer(bo, &ghost_obj, false);
>> if (ret)
>> return ret;
>> @@ -836,26 +848,51 @@ int ttm_bo_move_accel_cleanup(struct
>> ttm_buffer_object *bo,
>> }
>> EXPORT_SYMBOL(ttm_bo_move_accel_cleanup);
>> +/**
>> + * ttm_bo_pipeline_gutting - purge the contents of a bo
>> + * @bo: The buffer object
>> + *
>> + * Purge the contents of a bo, async if the bo is not idle.
>> + * After a successful call, the bo is left unpopulated in
>> + * system placement. The function may wait uninterruptible
>> + * for idle on OOM.
>> + *
>> + * Return: 0 if successful, negative error code on failure.
>> + */
>> int ttm_bo_pipeline_gutting(struct ttm_buffer_object *bo)
>> {
>> static const struct ttm_place sys_mem = { .mem_type =
>> TTM_PL_SYSTEM };
>> struct ttm_buffer_object *ghost;
>> int ret;
>> - ret = ttm_buffer_object_transfer(bo, &ghost);
>> - if (ret)
>> - return ret;
>> + /* If already idle, no need for ghost object dance. */
>> + ret = ttm_bo_wait(bo, false, true);
>> + if (ret == -EBUSY) {
>> + ret = ttm_buffer_object_transfer(bo, &ghost, true);
>> + if (ret)
>> + return ret;
>
> When this is a shortcout to avoid work we should rather use the
> inverse notation.
>
> In other words something like that:
>
> if (ret != -EBUSY) {
> ttm_resource_free(bo, &bo->mem);
> ttm_resource_alloc(bo, &sys_mem, &bo->mem);
> ttm_tt_create()...
> return ret;
> }
OK.
>
>> - ret = dma_resv_copy_fences(&ghost->base._resv, bo->base.resv);
>> - /* Last resort, wait for the BO to be idle when we are OOM */
>> - if (ret)
>> - ttm_bo_wait(bo, false, false);
>> + ret = dma_resv_copy_fences(&ghost->base._resv, bo->base.resv);
>> + /* Last resort, wait for the BO to be idle when we are OOM */
>> + if (ret)
>> + ttm_bo_wait(bo, false, false);
>> - ttm_resource_alloc(bo, &sys_mem, &bo->mem);
>> - bo->ttm = NULL;
>> + dma_resv_unlock(&ghost->base._resv);
>> + ttm_bo_put(ghost);
>> + } else {
>> + if (!bo->ttm) {
>> + ret = ttm_tt_create(bo, true);
>> + if (ret)
>> + return ret;
>> + } else {
>> + ttm_tt_unpopulate(bo->bdev, bo->ttm);
>> + if (bo->type == ttm_bo_type_device)
>> + ttm_tt_mark_for_clear(bo->ttm);
>
> That's not legal, you can't unpopulate it when the BO is busy.
>
> Instead the TT object must be destroyed with the ghost and a new one
> created.
We've already verified that the bo is idle here, so we should be fine.
/Thomas
>
> Christian.
>
>> + }
>> + ttm_resource_free(bo, &bo->mem);
>> + }
>> - dma_resv_unlock(&ghost->base._resv);
>> - ttm_bo_put(ghost);
>> + ttm_resource_alloc(bo, &sys_mem, &bo->mem);
>> return 0;
>> }
>> diff --git a/drivers/gpu/drm/ttm/ttm_tt.c b/drivers/gpu/drm/ttm/ttm_tt.c
>> index 539e0232cb3b..0b1053e93db2 100644
>> --- a/drivers/gpu/drm/ttm/ttm_tt.c
>> +++ b/drivers/gpu/drm/ttm/ttm_tt.c
>> @@ -134,6 +134,11 @@ void ttm_tt_destroy_common(struct ttm_device
>> *bdev, struct ttm_tt *ttm)
>> }
>> EXPORT_SYMBOL(ttm_tt_destroy_common);
>> +void ttm_tt_mark_for_clear(struct ttm_tt *ttm)
>> +{
>> + ttm->page_flags |= TTM_PAGE_FLAG_ZERO_ALLOC;
>> +}
>> +
>> void ttm_tt_destroy(struct ttm_device *bdev, struct ttm_tt *ttm)
>> {
>> bdev->funcs->ttm_tt_destroy(bdev, ttm);
>> diff --git a/include/drm/ttm/ttm_tt.h b/include/drm/ttm/ttm_tt.h
>> index 134d09ef7766..91552c83ac79 100644
>> --- a/include/drm/ttm/ttm_tt.h
>> +++ b/include/drm/ttm/ttm_tt.h
>> @@ -157,6 +157,16 @@ int ttm_tt_populate(struct ttm_device *bdev,
>> struct ttm_tt *ttm, struct ttm_oper
>> */
>> void ttm_tt_unpopulate(struct ttm_device *bdev, struct ttm_tt *ttm);
>> +/**
>> + * ttm_tt_mark_for_clear - Mark pages for clearing on populate.
>> + *
>> + * @ttm: Pointer to the ttm_tt structure
>> + *
>> + * Marks pages for clearing so that the next time the page vector is
>> + * populated, the pages will be cleared.
>> + */
>> +void ttm_tt_mark_for_clear(struct ttm_tt *ttm);
>> +
>> void ttm_tt_mgr_init(unsigned long num_pages, unsigned long
>> num_dma32_pages);
>> #if IS_ENABLED(CONFIG_AGP)
>
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