On Tue, Mar 15, 2022 at 09:45:20AM +0000, Tvrtko Ursulin wrote:
>
> On 15/03/2022 07:28, Kasireddy, Vivek wrote:
> > Hi Tvrtko, Daniel,
> >
> > >
> > > On 11/03/2022 09:39, Daniel Vetter wrote:
> > > > On Mon, 7 Mar 2022 at 21:38, Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx> 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.
> > > > >
> > > > > v7: (Ville)
> > > > > - Drop the HAS_GMCH(i915), DISPLAY_VER(i915) < 11 and
> > > > > size < ggtt->mappable_end / 4 checks.
> > > > > - Drop the redundant check that is based on previous heuristic.
> > > > >
> > > > > v8:
> > > > > - Make sure that we are holding the mutex associated with ggtt vm
> > > > > as we traverse the hole nodes.
> > > > >
> > > > > v9: (Tvrtko)
> > > > > - Use mutex_lock_interruptible_nested() instead of mutex_lock().
> > > > >
> > > > > 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>
> > > > > Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > > > > Signed-off-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
> > > > > ---
> > > > > drivers/gpu/drm/i915/i915_gem.c | 128 +++++++++++++++++++++++---------
> > > > > 1 file changed, 94 insertions(+), 34 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > > > > index 9747924cc57b..e0d731b3f215 100644
> > > > > --- a/drivers/gpu/drm/i915/i915_gem.c
> > > > > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > > > > @@ -49,6 +49,7 @@
> > > > > #include "gem/i915_gem_pm.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"
> > > > > @@ -882,6 +883,96 @@ 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)
> > > >
> > > > Tvrtko asked me to ack the first patch, but then I looked at this and
> > > > started wondering.
> > > >
> > > > Conceptually this doesn't pass the smell test. What if we have
> > > > multiple per-crtc buffers? Multiple planes on the same crtc? What if
> > > > the app does triple buffer? You'll be forever busy tuning this
> > > > heuristics, which can't fundamentally be fixed I think. The old "half
> > > > of mappable" heuristic isn't really better, but at least it was dead
> > > > simple.
> > > >
> > > > Imo what we need here is a change in approach:
> > > > 1. Check whether the useable view for scanout exists already. If yes,
> > > > use that. This should avoid the constant unbinding stalls.
> > > > 2. Try to in buffer to mappabley, but without evicting anything (so
> > > > not the non-blocking thing)
> > > > 3. Pin the buffer with the most lenient approach
> > > >
> > > > Even the non-blocking interim stage is dangerous, since it'll just
> > > > result in other buffers (e.g. when triple-buffering) getting unbound
> > > > and we're back to the same stall. Note that this could have an impact
> > > > on cpu rendering compositors, where we might end up relying a lot more
> > > > partial views. But as long as we are a tad more aggressive (i.e. the
> > > > non-blocking binding) in the mmap path that should work out to keep
> > > > everything balanced, since usually you render first before you display
> > > > anything. And so the buffer should end up in the ideal place.
> > > >
> > > > I'd try to first skip the 2. step since I think it'll require a bit of
> > > > work, and frankly I don't think we care about the potential fallout.
> > >
> > > To be sure I understand, you propose to stop trying to pin mappable by default. Ie. stop
> > > respecting this comment from i915_gem_object_pin_to_display_plane:
> > >
> > > /*
> > > * As the user may map the buffer once pinned in the display plane
> > > * (e.g. libkms for the bootup splash), we have to ensure that we
> > > * always use map_and_fenceable for all scanout buffers. However,
> > > * it may simply be too big to fit into mappable, in which case
> > > * put it anyway and hope that userspace can cope (but always first
> > > * try to preserve the existing ABI).
> > > */
> > [Kasireddy, Vivek] Digging further, this is what the commit message that added
> > the above comment says:
> > commit 2efb813d5388e18255c54afac77bd91acd586908
> > Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > Date: Thu Aug 18 17:17:06 2016 +0100
> >
> > drm/i915: Fallback to using unmappable memory for scanout
> >
> > The existing ABI says that scanouts are pinned into the mappable region
> > so that legacy clients (e.g. old Xorg or plymouthd) can write directly
> > into the scanout through a GTT mapping. However if the surface does not
> > fit into the mappable region, we are better off just trying to fit it
> > anywhere and hoping for the best. (Any userspace that is capable of
> > using ginormous scanouts is also likely not to rely on pure GTT
> > updates.) With the partial vma fault support, we are no longer
> > restricted to only using scanouts that we can pin (though it is still
> > preferred for performance reasons and for powersaving features like
> > FBC).
> >
> > >
> > > By a quick look, for this case it appears we would end up creating partial views for CPU
> > > access (since the normal mapping would be busy/unpinnable). Worst case for this is to
> > > create a bunch of 1MiB VMAs so something to check would be how long those persist in
> > > memory before they get released. Or perhaps the bootup splash use case is not common
> > > these days?
> > [Kasireddy, Vivek] AFAIK, Plymouth is still the default bootup splash service on Fedora,
> > Ubuntu and most other distributions. And, I took a quick look at it and IIUC, it (Plymouth's
> > drm plugin) seems to create a dumb FB, mmap and update it via the dirty_fb ioctl. This
> > would not to be a problem on ADL-S where there is space in mappable for one 8K FB.
> >
>
> FBC is a good point - correct me if I am wrong, but if we dropped trying to
> map in aperture by default it looks like we would lose it and that would be
> a significant power regression. In which case it doesn't seem like that
> would be an option.
FBC fence is only required for frontbuffer hw tracking, which is another
thing that's somewhere between "meh" and "we should just sunset set it
right away". I think that work has even been done.
So I wouldn't worry about this.
If you are worried, then I'd check with display folks whether we need
a platform based cut-off for this heuristics.
> Which I think leaves us with _some_ heuristics in any case.
>
> 1) N-holes heuristics.
>
> 2) Don't ever try PIN_MAPPABLE for framebuffers larger than some percentage
> of aperture.
>
> Could this solve the 8k issue, most of the time, maybe? Could the current
> "aperture / 2" test be expressed generically in some terms? Like "(aperture
> - 10% (or some absolute value)) / 2" to account for non-fb objects? I forgot
> what you said the relationship between aperture size and 8k fb size was.
>
> 3) Don't evict for PIN_MAPPABLE mismatches when
> i915_gem_object_ggtt_pin_ww->i915_vma_misplaced is called on behalf of
> i915_gem_object_pin_to_display_plane. Assumption being if we ended up with a
> non-mappable fb to start with, we must not try to re-bind it or we risk
> ping-pong latencies.
>
> The last would I guess need to distinguish between PIN_MAPPABLE passed in
> versus opportunistically added by i915_gem_object_pin_to_display_plane.
>
> How intrusive would it be to implement this option I am not sure without
> trying myself.
This won't work, see my initial mail. All you need is triple buffering (or
multiple per-crtc buffers that flip)
1. fb A gets pinned as mappable
2. fb B gets pinned as mappable, fb A is unpinned
3. fb C gets pinned as mappable, we don't have space and end up evicting
fb A
Repeat, and you have exactly the same old eviction loop as with two
buffers. Not good.
Therefore for this to work we don't just need to make sure that we don't
move our own buffer, but also that we don't move any other buffer.
The downside of that is that if a buffer is ever misplaced as mappable, we
never fix up that mistake (at least not until the application entirely
destroys all the involved fb and bo). I think that's acceptable, but
definitely deserves a comment.
Cheers, Daniel
>
> > Given this, do you think it would work if we just preserve the existing behavior and
> > tweak the heuristic introduced in this patch to look for space in aperture for only
> > one FB instead of two? Or, is there no good option for solving this issue other than
> > to create 1MB VMAs?
>
> I did not get how having one hole would solve the issue. Wouldn't it still
> hit the re-bind ping-pong? Or there isn't even a single hole for 8k fb
> typically?
>
> Regards,
>
> Tvrtko
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
