[Intel-gfx] [PATCH v6 2/2] drm/i915/gem: Don't try to map and fence large scanout buffers (v9)
Kasireddy, Vivek
vivek.kasireddy at intel.com
Thu Mar 17 07:08:25 UTC 2022
Hi Tvrtko,
>
> On 16/03/2022 07:37, Kasireddy, Vivek wrote:
> > Hi Tvrtko,
> >
> >>
> >> 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 at intel.com>
> 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.
> >>>>>>
> >>>>>> 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 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>
> >>>>>> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >>>>>> Signed-off-by: Vivek Kasireddy <vivek.kasireddy at intel.com>
> >>>>>> ---
> >>>>>> 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 at chris-wilson.co.uk>
> >>> 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.
> > [Kasireddy, Vivek] Ok, makes sense.
> >
> >>
> >> 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.
> > [Kasireddy, Vivek] I suspect I might be missing something, but could it not be
> > as simple as below:
> > @@ -940,7 +940,8 @@ i915_gem_object_ggtt_pin_ww(struct drm_i915_gem_object
> *obj,
> > return ERR_PTR(-ENOSPC);
> >
> > if (flags & PIN_MAPPABLE &&
> > - vma->fence_size > ggtt->mappable_end / 2)
> > + (vma->fence_size > ggtt->mappable_end / 2 ||
> > + !i915_vma_is_map_and_fenceable(vma)))
> > return ERR_PTR(-ENOSPC);
> > }
>
> Looks like this would work...
>
> >>
> >>> 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?
> > [Kasireddy, Vivek] IIUC, Mesa gives Weston a max of 4 backbuffers but it
> > almost always uses only 2 except when it needs to share the FB -- with a plugin
> > such as "remoting" for desktop streaming.
> > Given the common use-case, lets assume there are two 8K FBs: FB1 and FB2
> > FB1 is mappable/fenceable and therefore not misplaced.
> > FB2 is NOT mappable and hence identified as misplaced
> > (because it fails the check
> > (flags & PIN_MAPPABLE && !i915_vma_is_map_and_fenceable(vma))
> >
> > As you suggest in 3) above the goal is to ensure that FB2 does not get evicted
> > when we try to pin with PIN_MAPABBLE -- after it gets identified as misplaced.
> > Or, alternatively, when we pin with PIN_MAPABBLE, we could just check to
> > see if there is space in aperture for only FB2 (N = 1) and return early -- before
> > even getting to i915_vma_misplaced(). As you can see, we avoid the ping-pong
> > issue in both these cases.
>
> ... got it, yes, it seems both approaches works for this use case.
>
> Not sure that I have a preference between the two approaches at this point.
>
> Both would be behind a "PIN_MAPPABLE && PIN_NONBLOCK" check, so both
> would only apply to opportunistic PIN_MAPPABLE attempts. That is, any
> caller who only passes PIN_MAPPABLE would be unaffected which is what we
> want.
>
> The extra i915_vma_is_map_and_fenceable check I guess is simpler and
> self-contained. I assume you have a test setup and can try it out to
> check it really works?
[Kasireddy, Vivek] Yes, it works; my testcase just involves running Weston
with a mode of 8K at 60 on ADL-S and checking the FPS of the sample client
weston-simple-egl. With the fix included, the perf improves to 59 FPS from
40 FPS. I'll send out a new patch for review soon.
Oh, btw, do you think it is now pointless to merge the drm/mm patch that adds
the iterator given that we'd no longer have the i915 patch that uses it anymore?
Thanks,
Vivek
>
> > The current version of this patch -- when running Weston -- puts both FB1
> > and FB2 (N = 2) outside of aperture although there may be space for FB1.
> > I don't think this makes sense anymore given Plymouth's single-buffer
> > use-case that uses dirtyfb ioctl.
>
> Yes agreed, it sounds preferable to preserve the current behaviour there.
>
> Regards,
>
> Tvrtko
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