[Add Hugh]
On Fri 09-06-17 12:03:50, Chris Wilson wrote:
> When we are called to relieve mempressue via the shrinker, the only way
> we can make progress is either by discarding unwanted pages (those
> objects that userspace has marked MADV_DONTNEED) or by reclaiming the
> dirty objects via swap. As we know that is the only way to make further
> progress, we can initiate the writeback as we invalidate the objects.
> This means the objects we put onto the inactive anon lru list are
> already marked for reclaim+writeback and so will trigger a wait upon the
> writeback inside direct reclaim, greatly improving the success rate of
> direct reclaim on i915 objects.
>
> The corollary is that we may start a slow swap on opportunistic
> mempressure from the likes of the compaction + migration kthreads. This
> is limited by those threads only being allowed to shrink idle pages, but
> also that if we reactivate the page before it is swapped out by gpu
> activity, we only page the cost of repinning the page. The cost is most
> felt when an object is reused after mempressure, which hopefully
> excludes the latency sensitive tasks (as we are just extending the
> impact of swap thrashing to them).
I am not sure you can start writeback on shmem while it is not in the
swapcache. Hugh?
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> ---
> drivers/gpu/drm/i915/i915_drv.h | 2 +-
> drivers/gpu/drm/i915/i915_gem.c | 27 ++--------------
> drivers/gpu/drm/i915/i915_gem_shrinker.c | 55 +++++++++++++++++++++++++++++++-
> 3 files changed, 57 insertions(+), 27 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 602fb3324484..3ffe6504b391 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -3332,7 +3332,7 @@ enum i915_mm_subclass { /* lockdep subclass for obj->mm.lock */
>
> void __i915_gem_object_put_pages(struct drm_i915_gem_object *obj,
> enum i915_mm_subclass subclass);
> -void __i915_gem_object_invalidate(struct drm_i915_gem_object *obj);
> +void __i915_gem_object_truncate(struct drm_i915_gem_object *obj);
>
> enum i915_map_type {
> I915_MAP_WB = 0,
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 31cbe78171a9..1de94a8399a2 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2176,8 +2176,7 @@ i915_gem_mmap_gtt_ioctl(struct drm_device *dev, void *data,
> }
>
> /* Immediately discard the backing storage */
> -static void
> -i915_gem_object_truncate(struct drm_i915_gem_object *obj)
> +void __i915_gem_object_truncate(struct drm_i915_gem_object *obj)
> {
> i915_gem_object_free_mmap_offset(obj);
>
> @@ -2194,28 +2193,6 @@ i915_gem_object_truncate(struct drm_i915_gem_object *obj)
> obj->mm.pages = ERR_PTR(-EFAULT);
> }
>
> -/* Try to discard unwanted pages */
> -void __i915_gem_object_invalidate(struct drm_i915_gem_object *obj)
> -{
> - struct address_space *mapping;
> -
> - lockdep_assert_held(&obj->mm.lock);
> - GEM_BUG_ON(obj->mm.pages);
> -
> - switch (obj->mm.madv) {
> - case I915_MADV_DONTNEED:
> - i915_gem_object_truncate(obj);
> - case __I915_MADV_PURGED:
> - return;
> - }
> -
> - if (obj->base.filp == NULL)
> - return;
> -
> - mapping = obj->base.filp->f_mapping,
> - invalidate_mapping_pages(mapping, 0, (loff_t)-1);
> -}
> -
> static void
> i915_gem_object_put_pages_gtt(struct drm_i915_gem_object *obj,
> struct sg_table *pages)
> @@ -4212,7 +4189,7 @@ i915_gem_madvise_ioctl(struct drm_device *dev, void *data,
>
> /* if the object is no longer attached, discard its backing storage */
> if (obj->mm.madv == I915_MADV_DONTNEED && !obj->mm.pages)
> - i915_gem_object_truncate(obj);
> + __i915_gem_object_truncate(obj);
>
> args->retained = obj->mm.madv != __I915_MADV_PURGED;
> mutex_unlock(&obj->mm.lock);
> diff --git a/drivers/gpu/drm/i915/i915_gem_shrinker.c b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> index 1032f98add11..9f68304d6862 100644
> --- a/drivers/gpu/drm/i915/i915_gem_shrinker.c
> +++ b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> @@ -127,6 +127,59 @@ static bool unsafe_drop_pages(struct drm_i915_gem_object *obj)
> return !READ_ONCE(obj->mm.pages);
> }
>
> +static void __start_writeback(struct drm_i915_gem_object *obj)
> +{
> + struct address_space *mapping;
> + struct writeback_control wbc = {
> + .sync_mode = WB_SYNC_NONE,
> + .nr_to_write = SWAP_CLUSTER_MAX,
> + .range_start = 0,
> + .range_end = LLONG_MAX,
> + .for_reclaim = 1,
> + };
> + unsigned long i;
> +
> + lockdep_assert_held(&obj->mm.lock);
> + GEM_BUG_ON(obj->mm.pages);
> +
> + switch (obj->mm.madv) {
> + case I915_MADV_DONTNEED:
> + __i915_gem_object_truncate(obj);
> + case __I915_MADV_PURGED:
> + return;
> + }
> +
> + if (!obj->base.filp)
> + return;
> +
> + /* Force any other users of this object to refault */
> + mapping = obj->base.filp->f_mapping;
> + unmap_mapping_range(mapping, 0, (loff_t)-1, 0);
> +
> + /* Begin writeback on each dirty page */
> + for (i = 0; i < obj->base.size >> PAGE_SHIFT; i++) {
> + struct page *page;
> +
> + page = find_lock_entry(mapping, i);
> + if (!page || radix_tree_exceptional_entry(page))
> + continue;
> +
> + if (!page_mapped(page) && clear_page_dirty_for_io(page)) {
> + int ret;
> +
> + SetPageReclaim(page);
> + ret = mapping->a_ops->writepage(page, &wbc);
> + if (!PageWriteback(page))
> + ClearPageReclaim(page);
> + if (!ret)
> + goto put;
> + }
> + unlock_page(page);
> +put:
> + put_page(page);
> + }
> +}
> +
> /**
> * i915_gem_shrink - Shrink buffer object caches
> * @dev_priv: i915 device
> @@ -239,7 +292,7 @@ i915_gem_shrink(struct drm_i915_private *dev_priv,
> mutex_lock_nested(&obj->mm.lock,
> I915_MM_SHRINKER);
> if (!obj->mm.pages) {
> - __i915_gem_object_invalidate(obj);
> + __start_writeback(obj);
> list_del_init(&obj->global_link);
> count += obj->base.size >> PAGE_SHIFT;
> }
> --
> 2.11.0
>
--
Michal Hocko
SUSE Labs
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