[Intel-gfx] [PATCH 2/2] drm/i915/gem: Don't try to map and fence large scanout buffers (v6)
Tvrtko Ursulin
tvrtko.ursulin at linux.intel.com
Mon Feb 7 11:47:16 UTC 2022
On 07/02/2022 10:58, Ville Syrjälä wrote:
> On Thu, Feb 03, 2022 at 05:22:10PM -0800, Vivek Kasireddy wrote:
>> On platforms capable of allowing 8K (7680 x 4320) modes, pinning 2 or
>> more framebuffers/scanout buffers results in only one that is mappable/
>> fenceable. Therefore, pageflipping between these 2 FBs where only one
>> is mappable/fenceable creates latencies large enough to miss alternate
>> vblanks thereby producing less optimal framerate.
>>
>> This mainly happens because when i915_gem_object_pin_to_display_plane()
>> is called to pin one of the FB objs, the associated vma is identified
>> as misplaced and therefore i915_vma_unbind() is called which unbinds and
>> evicts it. This misplaced vma gets subseqently pinned only when
>> i915_gem_object_ggtt_pin_ww() is called without PIN_MAPPABLE. This
>> results in a latency of ~10ms and happens every other vblank/repaint cycle.
>> Therefore, to fix this issue, we try to see if there is space to map
>> at-least two objects of a given size and return early if there isn't. This
>> would ensure that we do not try with PIN_MAPPABLE for any objects that
>> are too big to map thereby preventing unncessary unbind.
>>
>> Testcase:
>> Running Weston and weston-simple-egl on an Alderlake_S (ADLS) platform
>> with a 8K at 60 mode results in only ~40 FPS. Since upstream Weston submits
>> a frame ~7ms before the next vblank, the latencies seen between atomic
>> commit and flip event are 7, 24 (7 + 16.66), 7, 24..... suggesting that
>> it misses the vblank every other frame.
>>
>> Here is the ftrace snippet that shows the source of the ~10ms latency:
>> i915_gem_object_pin_to_display_plane() {
>> 0.102 us | i915_gem_object_set_cache_level();
>> i915_gem_object_ggtt_pin_ww() {
>> 0.390 us | i915_vma_instance();
>> 0.178 us | i915_vma_misplaced();
>> i915_vma_unbind() {
>> __i915_active_wait() {
>> 0.082 us | i915_active_acquire_if_busy();
>> 0.475 us | }
>> intel_runtime_pm_get() {
>> 0.087 us | intel_runtime_pm_acquire();
>> 0.259 us | }
>> __i915_active_wait() {
>> 0.085 us | i915_active_acquire_if_busy();
>> 0.240 us | }
>> __i915_vma_evict() {
>> ggtt_unbind_vma() {
>> gen8_ggtt_clear_range() {
>> 10507.255 us | }
>> 10507.689 us | }
>> 10508.516 us | }
>>
>> v2: Instead of using bigjoiner checks, determine whether a scanout
>> buffer is too big by checking to see if it is possible to map
>> two of them into the ggtt.
>>
>> v3 (Ville):
>> - Count how many fb objects can be fit into the available holes
>> instead of checking for a hole twice the object size.
>> - Take alignment constraints into account.
>> - Limit this large scanout buffer check to >= Gen 11 platforms.
>>
>> v4:
>> - Remove existing heuristic that checks just for size. (Ville)
>> - Return early if we find space to map at-least two objects. (Tvrtko)
>> - Slightly update the commit message.
>>
>> v5: (Tvrtko)
>> - Rename the function to indicate that the object may be too big to
>> map into the aperture.
>> - Account for guard pages while calculating the total size required
>> for the object.
>> - Do not subject all objects to the heuristic check and instead
>> consider objects only of a certain size.
>> - Do the hole walk using the rbtree.
>> - Preserve the existing PIN_NONBLOCK logic.
>> - Drop the PIN_MAPPABLE check while pinning the VMA.
>>
>> v6: (Tvrtko)
>> - Return 0 on success and the specific error code on failure to
>> preserve the existing behavior.
>>
>> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
>> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
>> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
>> Cc: Manasi Navare <manasi.d.navare at intel.com>
>> Signed-off-by: Vivek Kasireddy <vivek.kasireddy at intel.com>
>> ---
>> drivers/gpu/drm/i915/i915_gem.c | 120 ++++++++++++++++++++++++--------
>> 1 file changed, 90 insertions(+), 30 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
>> index e3a2c2a0e156..39f0d17550c3 100644
>> --- a/drivers/gpu/drm/i915/i915_gem.c
>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>> @@ -46,6 +46,7 @@
>> #include "gem/i915_gem_mman.h"
>> #include "gem/i915_gem_region.h"
>> #include "gem/i915_gem_userptr.h"
>> +#include "gem/i915_gem_tiling.h"
>> #include "gt/intel_engine_user.h"
>> #include "gt/intel_gt.h"
>> #include "gt/intel_gt_pm.h"
>> @@ -876,6 +877,92 @@ static void discard_ggtt_vma(struct i915_vma *vma)
>> spin_unlock(&obj->vma.lock);
>> }
>>
>> +static int
>> +i915_gem_object_fits_in_aperture(struct drm_i915_gem_object *obj,
>> + u64 alignment, u64 flags)
>> +{
>> + struct drm_i915_private *i915 = to_i915(obj->base.dev);
>> + struct i915_ggtt *ggtt = to_gt(i915)->ggtt;
>> + struct drm_mm_node *hole;
>> + u64 hole_start, hole_end, start, end;
>> + u64 fence_size, fence_alignment;
>> + unsigned int count = 0;
>> +
>> + /*
>> + * If the required space is larger than the available
>> + * aperture, we will not able to find a slot for the
>> + * object and unbinding the object now will be in
>> + * vain. Worse, doing so may cause us to ping-pong
>> + * the object in and out of the Global GTT and
>> + * waste a lot of cycles under the mutex.
>> + */
>> + if (obj->base.size > ggtt->mappable_end)
>> + return -E2BIG;
>> +
>> + /*
>> + * If NONBLOCK is set the caller is optimistically
>> + * trying to cache the full object within the mappable
>> + * aperture, and *must* have a fallback in place for
>> + * situations where we cannot bind the object. We
>> + * can be a little more lax here and use the fallback
>> + * more often to avoid costly migrations of ourselves
>> + * and other objects within the aperture.
>> + */
>> + if (!(flags & PIN_NONBLOCK))
>> + return 0;
>> +
>> + /*
>> + * We only consider objects whose size is at-least a quarter of
>> + * the aperture to be too big and subject them to the new
>> + * heuristic below.
>> + */
>> + if (obj->base.size < ggtt->mappable_end / 4)
>> + return 0;
>
> That seems a fairly arbitrary thing to put here. Maybe something the
> caller should check/specify?
I have no strong opinion on this one. In my mind I categorised it under
"is it a large framebuffer" heuristics. Previously it was less than one
half of aperture always okay, now one quarter, plus 2x hole check if
larger. Both are heuristics. I even mentioned earlier if 2x should be an
input parameter as well, but again, given it's not an exported function
couldn't really justify it.
>
>> +
>> + if (HAS_GMCH(i915) || DISPLAY_VER(i915) < 11 ||
>> + !i915_gem_object_is_framebuffer(obj))
>> + return 0;
>
> None of that seems appropriate for a generic gem function
> with this name.
It's not exported though, maybe remove i915_gem prefix to avoid any
ideas of it being generic?
Regards,
Tvrtko
>
>> +
>> + fence_size = i915_gem_fence_size(i915, obj->base.size,
>> + i915_gem_object_get_tiling(obj),
>> + i915_gem_object_get_stride(obj));
>> +
>> + if (i915_vm_has_cache_coloring(&ggtt->vm))
>> + fence_size += 2 * I915_GTT_PAGE_SIZE;
>> +
>> + fence_alignment = i915_gem_fence_alignment(i915, obj->base.size,
>> + i915_gem_object_get_tiling(obj),
>> + i915_gem_object_get_stride(obj));
>> + alignment = max_t(u64, alignment, fence_alignment);
>> +
>> + /*
>> + * Assuming this object is a large scanout buffer, we try to find
>> + * out if there is room to map at-least two of them. There could
>> + * be space available to map one but to be consistent, we try to
>> + * avoid mapping/fencing any of them.
>> + */
>> + drm_mm_for_each_suitable_hole(hole, &ggtt->vm.mm, 0, ggtt->mappable_end,
>> + fence_size, DRM_MM_INSERT_LOW) {
>> + hole_start = drm_mm_hole_node_start(hole);
>> + hole_end = hole_start + hole->hole_size;
>> +
>> + do {
>> + start = round_up(hole_start, alignment);
>> + end = min_t(u64, hole_end, ggtt->mappable_end);
>> +
>> + if (range_overflows(start, fence_size, end))
>> + break;
>> +
>> + if (++count >= 2)
>> + return 0;
>> +
>> + hole_start = start + fence_size;
>> + } while (1);
>> + }
>> +
>> + return -ENOSPC;
>> +}
>> +
>> struct i915_vma *
>> i915_gem_object_ggtt_pin_ww(struct drm_i915_gem_object *obj,
>> struct i915_gem_ww_ctx *ww,
>> @@ -891,36 +978,9 @@ i915_gem_object_ggtt_pin_ww(struct drm_i915_gem_object *obj,
>>
>> if (flags & PIN_MAPPABLE &&
>> (!view || view->type == I915_GGTT_VIEW_NORMAL)) {
>> - /*
>> - * If the required space is larger than the available
>> - * aperture, we will not able to find a slot for the
>> - * object and unbinding the object now will be in
>> - * vain. Worse, doing so may cause us to ping-pong
>> - * the object in and out of the Global GTT and
>> - * waste a lot of cycles under the mutex.
>> - */
>> - if (obj->base.size > ggtt->mappable_end)
>> - return ERR_PTR(-E2BIG);
>> -
>> - /*
>> - * If NONBLOCK is set the caller is optimistically
>> - * trying to cache the full object within the mappable
>> - * aperture, and *must* have a fallback in place for
>> - * situations where we cannot bind the object. We
>> - * can be a little more lax here and use the fallback
>> - * more often to avoid costly migrations of ourselves
>> - * and other objects within the aperture.
>> - *
>> - * Half-the-aperture is used as a simple heuristic.
>> - * More interesting would to do search for a free
>> - * block prior to making the commitment to unbind.
>> - * That caters for the self-harm case, and with a
>> - * little more heuristics (e.g. NOFAULT, NOEVICT)
>> - * we could try to minimise harm to others.
>> - */
>> - if (flags & PIN_NONBLOCK &&
>> - obj->base.size > ggtt->mappable_end / 2)
>> - return ERR_PTR(-ENOSPC);
>> + ret = i915_gem_object_fits_in_aperture(obj, alignment, flags);
>> + if (ret)
>> + return ERR_PTR(ret);
>> }
>>
>> new_vma:
>> --
>> 2.34.1
>
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