Re: [Intel-gfx] [PATCH v2 1/3] drm/i915: Introduce refcounted sg-tables

Thomas Hellström
2021-10-28T09:35:55Z

On 10/28/21 10:47, Matthew Auld wrote:
> On 28/10/2021 08:04, Thomas Hellström wrote:
>> On Wed, 2021-10-27 at 19:03 +0100, Matthew Auld wrote:
>>> On 27/10/2021 11:52, Thomas Hellström wrote:
>>>> As we start to introduce asynchronous failsafe object migration,
>>>> where we update the object state and then submit asynchronous
>>>> commands we need to record what memory resources are actually used
>>>> by various part of the command stream. Initially for three
>>>> purposes:
>>>>
>>>> 1) Error capture.
>>>> 2) Asynchronous migration error recovery.
>>>> 3) Asynchronous vma bind.
>>>>
>>>> At the time where these happens, the object state may have been
>>>> updated
>>>> to be several migrations ahead and object sg-tables discarded.
>>>>
>>>> In order to make it possible to keep sg-tables with memory resource
>>>> information for these operations, introduce refcounted sg-tables
>>>> that
>>>> aren't freed until the last user is done with them.
>>>>
>>>> The alternative would be to reference information sitting on the
>>>> corresponding ttm_resources which typically have the same lifetime
>>>> as
>>>> these refcountes sg_tables, but that leads to other awkward
>>>> constructs:
>>>> Due to the design direction chosen for ttm resource managers that
>>>> would
>>>> lead to diamond-style inheritance, the LMEM resources may sometimes
>>>> be
>>>> prematurely freed, and finally the subclassed struct ttm_resource
>>>> would
>>>> have to bleed into the asynchronous vma bind code.
>>>>
>>>> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>>>> ---
>>>>    drivers/gpu/drm/i915/gem/i915_gem_object.h    |   4 +-
>>>>    .../gpu/drm/i915/gem/i915_gem_object_types.h  |   3 +-
>>>>    drivers/gpu/drm/i915/gem/i915_gem_shmem.c     |  16 +-
>>>>    drivers/gpu/drm/i915/gem/i915_gem_ttm.c       | 188 +++++++++++--
>>>> -----
>>>>    drivers/gpu/drm/i915/i915_scatterlist.c       |  62 ++++--
>>>>    drivers/gpu/drm/i915/i915_scatterlist.h       |  76 ++++++-
>>>>    drivers/gpu/drm/i915/intel_region_ttm.c       |  15 +-
>>>>    drivers/gpu/drm/i915/intel_region_ttm.h       |   5 +-
>>>>    drivers/gpu/drm/i915/selftests/mock_region.c  |  12 +-
>>>>    9 files changed, 262 insertions(+), 119 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h
>>>> b/drivers/gpu/drm/i915/gem/i915_gem_object.h
>>>> index a5479ac7a4ad..c5ab364d4311 100644
>>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_object.h
>>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.h
>>>> @@ -624,8 +624,8 @@ struct sg_table *shmem_alloc_st(struct
>>>> drm_i915_private *i915,
>>>>                                  size_t size, struct
>>>> intel_memory_region *mr,
>>>>                                  struct address_space *mapping,
>>>>                                  unsigned int max_segment);
>>>> -void shmem_free_st(struct sg_table *st, struct address_space
>>>> *mapping,
>>>> -                  bool dirty, bool backup);
>>>> +void shmem_free_st_table(struct sg_table *st, struct address_space
>>>> *mapping,
>>>> +                        bool dirty, bool backup);
>>>
>>> s/st_table/sg_table/?
>>>
>>> I thought st was shorthand for sg_table? Maybe shmem_sg_free_table?
>>
>> Yes the naming is indeed a bit unfortunate here. To be consistent with
>> the core's sg_free_table(), I changed to
>> shmem_sg_free_table() / shmem_sg_alloc_table.
>>
>>>
>>>
>>>>    void __shmem_writeback(size_t size, struct address_space
>>>> *mapping);
>>>>       #ifdef CONFIG_MMU_NOTIFIER
>>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
>>>> b/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
>>>> index a4b69a43b898..604ed5ad77f5 100644
>>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
>>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_object_types.h
>>>> @@ -544,6 +544,7 @@ struct drm_i915_gem_object {
>>>>                   */
>>>>                  struct list_head region_link;
>>>>    +               struct i915_refct_sgt *rsgt;
>>>>                  struct sg_table *pages;
>>>>                  void *mapping;
>>>>    @@ -597,7 +598,7 @@ struct drm_i915_gem_object {
>>>>          } mm;
>>>>             struct {
>>>> -               struct sg_table *cached_io_st;
>>>> +               struct i915_refct_sgt *cached_io_rsgt;
>>>>                  struct i915_gem_object_page_iter get_io_page;
>>>>                  struct drm_i915_gem_object *backup;
>>>>                  bool created:1;
>>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
>>>> b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
>>>> index 01f332d8dbde..67c6bee695c7 100644
>>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
>>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
>>>> @@ -25,8 +25,8 @@ static void check_release_pagevec(struct pagevec
>>>> *pvec)
>>>>          cond_resched();
>>>>    }
>>>>    -void shmem_free_st(struct sg_table *st, struct address_space
>>>> *mapping,
>>>> -                  bool dirty, bool backup)
>>>> +void shmem_free_st_table(struct sg_table *st, struct address_space
>>>> *mapping,
>>>> +                        bool dirty, bool backup)
>>>>    {
>>>>          struct sgt_iter sgt_iter;
>>>>          struct pagevec pvec;
>>>> @@ -49,7 +49,6 @@ void shmem_free_st(struct sg_table *st, struct
>>>> address_space *mapping,
>>>>                  check_release_pagevec(&pvec);
>>>>             sg_free_table(st);
>>>> -       kfree(st);
>>>>    }
>>>>       struct sg_table *shmem_alloc_st(struct drm_i915_private *i915,
>>>> @@ -171,7 +170,8 @@ struct sg_table *shmem_alloc_st(struct
>>>> drm_i915_private *i915,
>>>>    err_sg:
>>>>          sg_mark_end(sg);
>>>>          if (sg != st->sgl) {
>>>> -               shmem_free_st(st, mapping, false, false);
>>>> +               shmem_free_st_table(st, mapping, false, false);
>>>> +               kfree(st);
>>>>          } else {
>>>>                  mapping_clear_unevictable(mapping);
>>>>                  sg_free_table(st);
>>>> @@ -254,7 +254,8 @@ static int shmem_get_pages(struct
>>>> drm_i915_gem_object *obj)
>>>>          return 0;
>>>>       err_pages:
>>>> -       shmem_free_st(st, mapping, false, false);
>>>> +       shmem_free_st_table(st, mapping, false, false);
>>>> +       kfree(st);
>>>>          /*
>>>>           * shmemfs first checks if there is enough memory to
>>>> allocate the page
>>>>           * and reports ENOSPC should there be insufficient, along
>>>> with the usual
>>>> @@ -374,8 +375,9 @@ void i915_gem_object_put_pages_shmem(struct
>>>> drm_i915_gem_object *obj, struct sg_
>>>>          if (i915_gem_object_needs_bit17_swizzle(obj))
>>>>                  i915_gem_object_save_bit_17_swizzle(obj, pages);
>>>>    -       shmem_free_st(pages, file_inode(obj->base.filp)->i_mapping,
>>>> -                     obj->mm.dirty, obj->mm.madv ==
>>>> I915_MADV_WILLNEED);
>>>> +       shmem_free_st_table(pages, file_inode(obj->base.filp)-
>>>>> i_mapping,
>>>> +                           obj->mm.dirty, obj->mm.madv ==
>>>> I915_MADV_WILLNEED);
>>>> +       kfree(pages);
>>>>          obj->mm.dirty = false;
>>>>    }
>>>>    diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
>>>> b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
>>>> index 4fd2edb20dd9..6826e3859e18 100644
>>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
>>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
>>>> @@ -34,7 +34,7 @@
>>>>     * struct i915_ttm_tt - TTM page vector with additional private
>>>> information
>>>>     * @ttm: The base TTM page vector.
>>>>     * @dev: The struct device used for dma mapping and unmapping.
>>>> - * @cached_st: The cached scatter-gather table.
>>>> + * @cached_rsgt: The cached scatter-gather table.
>>>>     * @is_shmem: Set if using shmem.
>>>>     * @filp: The shmem file, if using shmem backend.
>>>>     *
>>>> @@ -47,7 +47,7 @@
>>>>    struct i915_ttm_tt {
>>>>          struct ttm_tt ttm;
>>>>          struct device *dev;
>>>> -       struct sg_table *cached_st;
>>>> +       struct i915_refct_sgt cached_rsgt;
>>>>             bool is_shmem;
>>>>          struct file *filp;
>>>> @@ -221,14 +221,10 @@ static int i915_ttm_tt_shmem_populate(struct
>>>> ttm_device *bdev,
>>>>          if (IS_ERR(st))
>>>>                  return PTR_ERR(st);
>>>>    -       err = dma_map_sg_attrs(i915_tt->dev,
>>>> -                              st->sgl, st->nents,
>>>> -                              DMA_BIDIRECTIONAL,
>>>> -                              DMA_ATTR_SKIP_CPU_SYNC);
>>>> -       if (err <= 0) {
>>>> -               err = -EINVAL;
>>>> +       err = dma_map_sgtable(i915_tt->dev, st, DMA_BIDIRECTIONAL,
>>>> +                             DMA_ATTR_SKIP_CPU_SYNC);
>>>> +       if (err)
>>>>                  goto err_free_st;
>>>> -       }
>>>>             i = 0;
>>>>          for_each_sgt_page(page, sgt_iter, st)
>>>> @@ -237,11 +233,15 @@ static int i915_ttm_tt_shmem_populate(struct
>>>> ttm_device *bdev,
>>>>          if (ttm->page_flags & TTM_TT_FLAG_SWAPPED)
>>>>                  ttm->page_flags &= ~TTM_TT_FLAG_SWAPPED;
>>>>    -       i915_tt->cached_st = st;
>>>> +       i915_tt->cached_rsgt.table = *st;
>>>> +       kfree(st);
>>>
>>> Will it work if the above just operates on the rsgt.table?
>>
>> I'll change the shmem utility here to not allocate the struct sg_table
>> and then we can operate on it directly.
>>
>>>
>>>> +
>>>>          return 0;
>>>>       err_free_st:
>>>> -       shmem_free_st(st, filp->f_mapping, false, false);
>>>> +       shmem_free_st_table(st, filp->f_mapping, false, false);
>>>> +       kfree(st);
>>>> +
>>>>          return err;
>>>>    }
>>>>    @@ -249,16 +249,29 @@ static void
>>>> i915_ttm_tt_shmem_unpopulate(struct ttm_tt *ttm)
>>>>    {
>>>>          struct i915_ttm_tt *i915_tt = container_of(ttm,
>>>> typeof(*i915_tt), ttm);
>>>>          bool backup = ttm->page_flags & TTM_TT_FLAG_SWAPPED;
>>>> +       struct sg_table *st = &i915_tt->cached_rsgt.table;
>>>>    -       dma_unmap_sg(i915_tt->dev, i915_tt->cached_st->sgl,
>>>> -                    i915_tt->cached_st->nents,
>>>> -                    DMA_BIDIRECTIONAL);
>>>> +       if (st->sgl)
>>>
>>> Can we ever hit this? I would have presumed not? Below also.
>>
>> Yes, here's where we typically free the scatterlist.
>
> What I meant to say: can the above ever not be true? i.e sgl == NULL

Hm, Yes I think it can. If we're populating a non-shmem ttm tt, that
sg-list is, IIRC allocated lazily on first use. Although I haven't
analyzed in detail whether it can actually be populated and then not
lazily allocated under the same lock.