[Intel-gfx] [PATCH v6 2/2] drm/i915/gem: Don't try to map and fence large scanout buffers (v9)
Tvrtko Ursulin
tvrtko.ursulin at linux.intel.com
Tue Mar 15 09:45:20 UTC 2022
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.
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.
> 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
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