On Mon, Feb 07, 2022 at 11:47:16AM +0000, Tvrtko Ursulin wrote:
>
> 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@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@xxxxxxxxxxxxxxx>
> >> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> >> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
> >> Cc: Manasi Navare <manasi.d.navare@xxxxxxxxx>
> >> Signed-off-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
> >> ---
> >> 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.
Is there any point in even having this extra check? If we
don't think checking this is worth the hassle then why call
the function at all?
>
> >
> >> +
> >> + 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?
These checks don't even seem to doing anything useful. HAS_GMCH should
already be covered by always setting PIN_MAPPABLE and hence O_NONBLOCK
is never even tried, the pre-icl vs. icl+ check should not exist at all
IMO, and if this is only called for framebuffers then why does the code
pretend that is not the case?
So I would suggest just ditching all these checks, and then the function
even does what it says on the tin.
--
Ville Syrjälä
Intel
