Re: [PATCH 2/2] Revert "drm/xe/lnl: Offload system clear page activity to GPU"

[Matthew Brost <matthew.brost@intel.com>]

On Tue, Aug 27, 2024 at 05:49:10PM +0200, Nirmoy Das wrote:
> This optimization relied on having to clear CCS on allocations.
> If there is no need to clear CCS on allocations then this would mostly
> help in reducing CPU utilization.
> 
> Revert this patch because of:
> 1 Currently Xe can't do clear on free and using a invalid ttm flag,
> TTM_TT_FLAG_CLEARED_ON_FREE which could poison global ttm pool on
> multi-device setup.
> 
> 2 BO with WB caching type can't have compression enabled so there
> is no need to do clear CCS for such allocation. Reducing CPU
> utilization is a plus but latency is more important.
> 
> Without this patch:
> UsmMemoryAllocation(api=l0 type=Host size=4KB)  113.1 us
> UsmMemoryAllocation(api=l0 type=Host size=1GB)  91452.6 us
> 13.93% api_overhead_be  [kernel.kallsyms] [k] pages_are_mergeable
> 7.59%  api_overhead_be  [kernel.kallsyms] [k] __lock_acquire
> 3.86%  api_overhead_be  [kernel.kallsyms] [k] lock_is_held_type
> 3.39%  api_overhead_be  [kernel.kallsyms] [k] rcu_is_watching
> 3.37%  api_overhead_be  [xe]              [k] emit_pte
> 
> With this patch:
> UsmMemoryAllocation(api=l0 type=Host size=4KB) 91.7 us
> UsmMemoryAllocation(api=l0 type=Host size=1GB) 89486.5 us
> 91.86%  api_overhead_be [kernel.kallsyms [k] clear_page_erms
> 1.81%  api_overhead_be  [kernel.kallsyms [k] pages_are_mergeable
> 0.99%  api_overhead_be  [kernel.kallsyms [k] get_page_from_freelist
> 0.63%  api_overhead_be  [kernel.kallsyms [k] __free_pages_ok
> 0.50%  api_overhead_be  [kernel.kallsyms [k] __lock_acquire
> 

Cool stats. How did you collect these. I'll likely need to collect
similar stats very soon for some of the working I'm doing.

> For larger memory the delta is very low so it is likely worth
> doing gpu based system page clear that can be reconsidered in the
> future.
> 
> This reverts commit 23683061805be368c8d1c7e7ff52abc470cac275 with
> minor conflict fixes.
> 
> Cc: Matthew Auld <matthew.auld@intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>

Anyways:
Reviewed-by: Matthew Brost <matthew.brost@intel.com>

> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_bo.c           | 26 ++------------------------
>  drivers/gpu/drm/xe/xe_device_types.h |  2 --
>  drivers/gpu/drm/xe/xe_ttm_sys_mgr.c  | 12 ------------
>  3 files changed, 2 insertions(+), 38 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index 24701272e3af..fd4b2b2dfd8d 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -397,14 +397,6 @@ static struct ttm_tt *xe_ttm_tt_create(struct ttm_buffer_object *ttm_bo,
>  		caching = ttm_uncached;
>  	}
>  
> -	/*
> -	 * If the device can support gpu clear system pages then set proper ttm
> -	 * flag. Zeroed pages are only required for ttm_bo_type_device so
> -	 * unwanted data is not leaked to userspace.
> -	 */
> -	if (ttm_bo->type == ttm_bo_type_device && xe->mem.gpu_page_clear_sys)
> -		page_flags |= TTM_TT_FLAG_CLEARED_ON_FREE;
> -
>  	/* compression is not allowed for cached BO so ccs clear can be skipped. */
>  	tt->skip_ccs_clear = caching == ttm_cached;
>  	err = ttm_tt_init(&tt->ttm, &bo->ttm, page_flags, caching, extra_pages);
> @@ -428,10 +420,6 @@ static int xe_ttm_tt_populate(struct ttm_device *ttm_dev, struct ttm_tt *tt,
>  	if (tt->page_flags & TTM_TT_FLAG_EXTERNAL)
>  		return 0;
>  
> -	/* Clear TTM_TT_FLAG_ZERO_ALLOC when GPU is set to clear system pages */
> -	if (tt->page_flags & TTM_TT_FLAG_CLEARED_ON_FREE)
> -		tt->page_flags &= ~TTM_TT_FLAG_ZERO_ALLOC;
> -
>  	err = ttm_pool_alloc(&ttm_dev->pool, tt, ctx);
>  	if (err)
>  		return err;
> @@ -677,16 +665,8 @@ static int xe_bo_move(struct ttm_buffer_object *ttm_bo, bool evict,
>  	bool handle_system_ccs = (!IS_DGFX(xe) && xe_bo_needs_ccs_pages(bo) &&
>  				  ttm && ttm_tt_is_populated(ttm) &&
>  				  !xe_tt->skip_ccs_clear) ? true : false;
> -	bool clear_system_pages;
>  	int ret = 0;
>  
> -	/*
> -	 * Clear TTM_TT_FLAG_CLEARED_ON_FREE on bo creation path when
> -	 * moving to system as the bo doesn't have dma_mapping.
> -	 */
> -	if (!old_mem && ttm && !ttm_tt_is_populated(ttm))
> -		ttm->page_flags &= ~TTM_TT_FLAG_CLEARED_ON_FREE;
> -
>  	/* Bo creation path, moving to system or TT. */
>  	if ((!old_mem && ttm) && !handle_system_ccs) {
>  		if (new_mem->mem_type == XE_PL_TT)
> @@ -709,10 +689,8 @@ static int xe_bo_move(struct ttm_buffer_object *ttm_bo, bool evict,
>  	move_lacks_source = handle_system_ccs ? (!bo->ccs_cleared)  :
>  						(!mem_type_is_vram(old_mem_type) && !tt_has_data);
>  
> -	clear_system_pages = ttm && (ttm->page_flags & TTM_TT_FLAG_CLEARED_ON_FREE);
>  	needs_clear = (ttm && ttm->page_flags & TTM_TT_FLAG_ZERO_ALLOC) ||
> -		(!ttm && ttm_bo->type == ttm_bo_type_device) ||
> -		clear_system_pages;
> +		(!ttm && ttm_bo->type == ttm_bo_type_device);
>  
>  	if (new_mem->mem_type == XE_PL_TT) {
>  		ret = xe_tt_map_sg(ttm);
> @@ -824,7 +802,7 @@ static int xe_bo_move(struct ttm_buffer_object *ttm_bo, bool evict,
>  		if (move_lacks_source) {
>  			u32 flags = 0;
>  
> -			if (mem_type_is_vram(new_mem->mem_type) || clear_system_pages)
> +			if (mem_type_is_vram(new_mem->mem_type))
>  				flags |= XE_MIGRATE_CLEAR_FLAG_FULL;
>  			else if (handle_system_ccs)
>  				flags |= XE_MIGRATE_CLEAR_FLAG_CCS_DATA;
> diff --git a/drivers/gpu/drm/xe/xe_device_types.h b/drivers/gpu/drm/xe/xe_device_types.h
> index 4ecd620921a3..e73fb0c23932 100644
> --- a/drivers/gpu/drm/xe/xe_device_types.h
> +++ b/drivers/gpu/drm/xe/xe_device_types.h
> @@ -333,8 +333,6 @@ struct xe_device {
>  		struct xe_mem_region vram;
>  		/** @mem.sys_mgr: system TTM manager */
>  		struct ttm_resource_manager sys_mgr;
> -		/** @mem.gpu_page_clear_sys: clear system pages offloaded to GPU */
> -		bool gpu_page_clear_sys;
>  	} mem;
>  
>  	/** @sriov: device level virtualization data */
> diff --git a/drivers/gpu/drm/xe/xe_ttm_sys_mgr.c b/drivers/gpu/drm/xe/xe_ttm_sys_mgr.c
> index e0ac20f20758..9844a8edbfe1 100644
> --- a/drivers/gpu/drm/xe/xe_ttm_sys_mgr.c
> +++ b/drivers/gpu/drm/xe/xe_ttm_sys_mgr.c
> @@ -117,17 +117,5 @@ int xe_ttm_sys_mgr_init(struct xe_device *xe)
>  	ttm_resource_manager_init(man, &xe->ttm, gtt_size >> PAGE_SHIFT);
>  	ttm_set_driver_manager(&xe->ttm, XE_PL_TT, man);
>  	ttm_resource_manager_set_used(man, true);
> -
> -	/*
> -	 * On iGFX device with flat CCS, we clear CCS metadata, let's extend that
> -	 * and use GPU to clear pages as well.
> -	 *
> -	 * Disable this when init_on_free and/or init_on_alloc is on to avoid double
> -	 * zeroing pages with CPU and GPU.
> -	 */
> -	if (xe_device_has_flat_ccs(xe) && !IS_DGFX(xe) &&
> -	    !want_init_on_alloc(GFP_USER) && !want_init_on_free())
> -		xe->mem.gpu_page_clear_sys = true;
> -
>  	return drmm_add_action_or_reset(&xe->drm, ttm_sys_mgr_fini, xe);
>  }
> -- 
> 2.42.0
>