Re: [Intel-gfx] [PATCH 2/2] drm/i915/gem: Don't try to map and fence large scanout buffers (v6)

["Kasireddy, Vivek" <vivek.kasireddy@intel.com>]

Hi Tvrtko, Ville,

> On 07/02/2022 13:24, Ville Syrjälä wrote:
> > 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@linux.intel.com>
> >>>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> >>>> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> >>>> Cc: Manasi Navare <manasi.d.navare@intel.com>
> >>>> Signed-off-by: Vivek Kasireddy <vivek.kasireddy@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.
> >
> > 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?
> 
> The "/4" one? It was my suggestion to avoid the hole search if we can know based on size
> it cannot be a frame buffer that would be affected by the ping-ping problem. Granted that
> was before the rbtree hole search, when it was traversing the un-ordered linked list of
> holes. What is the correct size threshold I don't know.
> 
> >>>> +
> >>>> +	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
[Kasireddy, Vivek] I can drop the HAS_GMCH(i915) check given that it is redundant.

> > is never even tried, the pre-icl vs. icl+ check should not exist at
[Kasireddy, Vivek] My aim was to narrow down the list of situations in which the
ping-pong problem becomes more pronounced and may lead to performance 
issues. Therefore, I added the DISPLAY_VER(i915) check since 8K/bigjoiner is
feasible only on those newer platforms. 

> > all IMO, and if this is only called for framebuffers then why does the
> > code pretend that is not the case?
[Kasireddy, Vivek] Oh, I added the i915_gem_object_is_framebuffer() check after I 
found that there are other callers (for example, reloc_iomap() in i915_gem_execbuffer.c)
of i915_gem_object_ggtt_pin_ww() that may not be working on an fb.

Also, I figured size < ggtt->mappable_end / 4 or a similar check is needed as we do
not want to subject all FBs through this performance critical path. 

Thanks,
Vivek

> >
> > So I would suggest just ditching all these checks, and then the
> > function even does what it says on the tin.
> 
> Change log for v3 made me think at least some of this was your suggestion so I did not
> think about it further. :) No strong opinion either.
> 
> Regards,
> 
> Tvrtko