Re: [RFC PATCH 24/28] drm/xe: Basic SVM BO eviction

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

On Thu, Aug 29, 2024 at 12:14:53PM +0200, Daniel Vetter wrote:
> On Tue, Aug 27, 2024 at 07:48:57PM -0700, Matthew Brost wrote:
> > Wire xe_bo_move to GPUSVM migration to SRAM with trylocking of mmap
> > lock.
> > 
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >  drivers/gpu/drm/xe/xe_bo.c       | 35 +++++++++++++++++++++++++++++++-
> >  drivers/gpu/drm/xe/xe_bo_types.h |  3 +++
> >  drivers/gpu/drm/xe/xe_svm.c      |  2 ++
> >  drivers/gpu/drm/xe/xe_svm.h      | 13 ++++++++++++
> >  4 files changed, 52 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> > index ad804b6f9e84..ae71fcbe5380 100644
> > --- a/drivers/gpu/drm/xe/xe_bo.c
> > +++ b/drivers/gpu/drm/xe/xe_bo.c
> > @@ -25,6 +25,7 @@
> >  #include "xe_pm.h"
> >  #include "xe_preempt_fence.h"
> >  #include "xe_res_cursor.h"
> > +#include "xe_svm.h"
> >  #include "xe_trace_bo.h"
> >  #include "xe_ttm_stolen_mgr.h"
> >  #include "xe_vm.h"
> > @@ -250,6 +251,8 @@ int xe_bo_placement_for_flags(struct xe_device *xe, struct xe_bo *bo,
> >  static void xe_evict_flags(struct ttm_buffer_object *tbo,
> >  			   struct ttm_placement *placement)
> >  {
> > +	struct xe_bo *bo;
> > +
> >  	if (!xe_bo_is_xe_bo(tbo)) {
> >  		/* Don't handle scatter gather BOs */
> >  		if (tbo->type == ttm_bo_type_sg) {
> > @@ -261,6 +264,12 @@ static void xe_evict_flags(struct ttm_buffer_object *tbo,
> >  		return;
> >  	}
> >  
> > +	bo = ttm_to_xe_bo(tbo);
> > +	if (bo->flags & XE_BO_FLAG_SYSTEM_ALLOC) {
> > +		*placement = sys_placement;
> > +		return;
> > +	}
> > +
> >  	/*
> >  	 * For xe, sg bos that are evicted to system just triggers a
> >  	 * rebind of the sg list upon subsequent validation to XE_PL_TT.
> > @@ -758,6 +767,17 @@ static int xe_bo_move(struct ttm_buffer_object *ttm_bo, bool evict,
> >  		}
> >  	}
> >  
> > +	if (!move_lacks_source && (bo->flags & XE_BO_FLAG_SYSTEM_ALLOC) &&
> > +	    new_mem->mem_type == XE_PL_SYSTEM) {
> > +		ret = xe_svm_range_evict(bo->range);
> > +		if (!ret) {
> > +			drm_dbg(&xe->drm, "Evict system allocator BO success\n");
> > +			ttm_bo_move_null(ttm_bo, new_mem);
> > +		}
> > +
> > +		goto out;
> > +	}
> > +
> >  	if (!move_lacks_source &&
> >  	    ((old_mem_type == XE_PL_SYSTEM && resource_is_vram(new_mem)) ||
> >  	     (mem_type_is_vram(old_mem_type) &&
> > @@ -1096,6 +1116,19 @@ static void xe_ttm_bo_delete_mem_notify(struct ttm_buffer_object *ttm_bo)
> >  	}
> >  }
> >  
> > +static bool xe_bo_eviction_valuable(struct ttm_buffer_object *ttm_bo,
> > +				    const struct ttm_place *place)
> > +{
> > +	struct xe_bo *bo = ttm_to_xe_bo(ttm_bo);
> > +
> > +	/* Do not evict SVMs before having a binding */
> > +	if (bo->flags & XE_BO_FLAG_SYSTEM_ALLOC &&
> > +	    !xe_svm_range_has_vram_binding(bo->range))
> > +		return false;
> > +
> > +	return ttm_bo_eviction_valuable(ttm_bo, place);
> > +}
> > +
> >  const struct ttm_device_funcs xe_ttm_funcs = {
> >  	.ttm_tt_create = xe_ttm_tt_create,
> >  	.ttm_tt_populate = xe_ttm_tt_populate,
> > @@ -1106,7 +1139,7 @@ const struct ttm_device_funcs xe_ttm_funcs = {
> >  	.io_mem_reserve = xe_ttm_io_mem_reserve,
> >  	.io_mem_pfn = xe_ttm_io_mem_pfn,
> >  	.release_notify = xe_ttm_bo_release_notify,
> > -	.eviction_valuable = ttm_bo_eviction_valuable,
> > +	.eviction_valuable = xe_bo_eviction_valuable,
> >  	.delete_mem_notify = xe_ttm_bo_delete_mem_notify,
> >  };
> >  
> > diff --git a/drivers/gpu/drm/xe/xe_bo_types.h b/drivers/gpu/drm/xe/xe_bo_types.h
> > index 2ed558ac2264..4523b033417c 100644
> > --- a/drivers/gpu/drm/xe/xe_bo_types.h
> > +++ b/drivers/gpu/drm/xe/xe_bo_types.h
> > @@ -16,6 +16,7 @@
> >  #include "xe_ggtt_types.h"
> >  
> >  struct xe_device;
> > +struct xe_svm_range;
> >  struct xe_vm;
> >  
> >  #define XE_BO_MAX_PLACEMENTS	3
> > @@ -47,6 +48,8 @@ struct xe_bo {
> >  	struct ttm_bo_kmap_obj kmap;
> >  	/** @pinned_link: link to present / evicted list of pinned BO */
> >  	struct list_head pinned_link;
> > +	/** @range: SVM range for BO */
> > +	struct xe_svm_range *range;
> >  #ifdef CONFIG_PROC_FS
> >  	/**
> >  	 * @client: @xe_drm_client which created the bo
> > diff --git a/drivers/gpu/drm/xe/xe_svm.c b/drivers/gpu/drm/xe/xe_svm.c
> > index fd8987e0a506..dc9810828c0a 100644
> > --- a/drivers/gpu/drm/xe/xe_svm.c
> > +++ b/drivers/gpu/drm/xe/xe_svm.c
> > @@ -531,6 +531,8 @@ static struct xe_bo *xe_svm_alloc_vram(struct xe_vm *vm, struct xe_tile *tile,
> >  			  range->base.va.start, ttm_bo_type_device,
> >  			  XE_BO_FLAG_VRAM_IF_DGFX(tile) |
> >  			  XE_BO_FLAG_SYSTEM_ALLOC | XE_BO_FLAG_SKIP_CLEAR);
> > +	if (!IS_ERR(bo))
> > +		bo->range = range;
> >  	xe_vm_unlock(vm);
> >  	if (IS_ERR(bo)) {
> >  		err = PTR_ERR(bo);
> > diff --git a/drivers/gpu/drm/xe/xe_svm.h b/drivers/gpu/drm/xe/xe_svm.h
> > index 3f432483a230..b9cf0e2500da 100644
> > --- a/drivers/gpu/drm/xe/xe_svm.h
> > +++ b/drivers/gpu/drm/xe/xe_svm.h
> > @@ -46,6 +46,19 @@ static inline bool xe_svm_range_has_dma_mapping(struct xe_svm_range *range)
> >  	return range->base.flags.has_dma_mapping;
> >  }
> >  
> > +static inline bool xe_svm_range_has_vram_binding(struct xe_svm_range *range)
> > +{
> > +	return xe_svm_range_in_vram(range) && range->tile_present;
> > +}
> > +
> > +static inline int xe_svm_range_evict(struct xe_svm_range *range)
> > +{
> > +	struct drm_gpusvm_ctx ctx = { .trylock_mmap = true, };
> 
> So even trying to acquire an mmap lock for eviction is I think a design
> bug for svm memory ranges. It's a bunch of physical memory, you have no
> idea how many mm/vma map it and which one you pick as the special one is
> fairly arbitrary.
> 

Let me drop whole trylock this and just evict via
drm_gpusvm_evict_to_sram / migrate_device_vma_range which does not
require the mmap. I add this code very recently per Matt Auld suggestion
and agree it makes the try locking unnecessary.

> So dont, eviction should entirely ignore va/mm issues at the top level
> like the migrate_device_range function does (maybe we need a
> scatter-gather version of that instead of just a range.
> 

I needed to add migrate_device_vma_range (might be a bad name) as VRAM
may be non-continuous pfns when memory pressure exists where as
migrate_device_range only supports continuous pfns.

> That function internally makes sure you're in sync with any vma/vm by:
> - installing migration ptes everywhere, which does the mmu_notifer dance
> - locking the pages to prevent other concurrent migration or other fun
>   stuff from happening
> - then restore ptes to something sensisble when it's all done
> 
> And it does that by looping over _all_ possible mappings of a page with
> the rmap_walk infrastructure.
> 
> The only reason when we need the mmap lock (or vma lock or whatever) is if
> we need to be coherent with other concurrent mm updates of a specific mm.
> That should only be the case when migrating to vram, where the gpusvm->mm
> is the special one, and when migrating to sram due to cpu faults, where
> the vmf->vma->mm is special (and might at best have a tenous relationship
> to the gpusvm->mm). But that's the only cases where a specific mm and vma
> have any relevance to svm vram allocations.
> 

Thanks for the info.

Matt

> -Sima
> 
> > +
> > +	return drm_gpusvm_migrate_to_sram(range->base.gpusvm, &range->base,
> > +					  &ctx);
> > +}
> > +
> >  #define xe_svm_notifier_lock(vm__)	\
> >  	drm_gpusvm_notifier_lock(&(vm__)->svm.gpusvm)
> >  
> > -- 
> > 2.34.1
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch