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dma-fence: Add a single fence fast path for fence merging

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Testing some workloads in two different scenarios, such as games running
under Gamescope on a Steam Deck, or vkcube under a Plasma desktop, shows
that in a significant portion of calls the dma_fence_unwrap_merge helper
is called with just a single unsignalled fence.

Therefore it is worthile to add a fast path for that case and so bypass
the memory allocation and insertion sort attempts.

Tested scenarios:

1) Hogwarts Legacy under Gamescope

~1500 calls per second to __dma_fence_unwrap_merge.

Percentages per number of fences buckets, before and after checking for
signalled status, sorting and flattening:

   N       Before      After
   0       0.85%
   1      69.80%        ->   The new fast path.
  2-9     29.36%        9%   (Ie. 91% of this bucket flattened to 1 fence)
 10-19
 20-40
  50+

2) Cyberpunk 2077 under Gamescope

~2400 calls per second.

   N       Before      After
   0       0.71%
   1      52.53%        ->    The new fast path.
  2-9     44.38%      50.60%  (Ie. half resolved to a single fence)
 10-19     2.34%
 20-40     0.06%
  50+

3) vkcube under Plasma

90 calls per second.

   N       Before      After
   0
   1
  2-9      100%         0%   (Ie. all resolved to a single fence)
 10-19
 20-40
  50+

In the case of vkcube all invocations in the 2-9 bucket were actually
just two input fences.

v2:
 * Correct local variable name and hold on to unsignaled reference. (Chistian)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Friedrich Vock <friedrich.vock@gmx.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241115102153.1980-4-tursulin@igalia.com
This commit is contained in:
Tvrtko Ursulin 2024-11-15 10:21:51 +00:00 committed by Christian König
parent 2ef1c8c5de
commit 178ada9d6e
1 changed files with 10 additions and 1 deletions
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11
drivers/dma-buf/dma-fence-unwrap.c
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@ -84,8 +84,8 @@ struct dma_fence *__dma_fence_unwrap_merge(unsigned int num_fences,
struct dma_fence **fences,
struct dma_fence_unwrap *iter)
{
struct dma_fence *tmp, *unsignaled = NULL, **array;
struct dma_fence_array *result;
struct dma_fence *tmp, **array;
ktime_t timestamp;
int i, j, count;
@ -94,6 +94,8 @@ struct dma_fence *__dma_fence_unwrap_merge(unsigned int num_fences,
for (i = 0; i < num_fences; ++i) {
dma_fence_unwrap_for_each(tmp, &iter[i], fences[i]) {
if (!dma_fence_is_signaled(tmp)) {
dma_fence_put(unsignaled);
unsignaled = dma_fence_get(tmp);
++count;
} else {
ktime_t t = dma_fence_timestamp(tmp);
@ -107,9 +109,16 @@ struct dma_fence *__dma_fence_unwrap_merge(unsigned int num_fences,
/*
* If we couldn't find a pending fence just return a private signaled
* fence with the timestamp of the last signaled one.
*
* Or if there was a single unsignaled fence left we can return it
* directly and early since that is a major path on many workloads.
*/
if (count == 0)
return dma_fence_allocate_private_stub(timestamp);
else if (count == 1)
return unsignaled;
dma_fence_put(unsignaled);
array = kmalloc_array(count, sizeof(*array), GFP_KERNEL);
if (!array)